A good guide is to price a staple comodity. how many loaves of bread would $7,000 buy, how much would it cost to buy that many today?
This is by far the more reasonable way to approach this question. One of the problems one runs into in determining what an amount of money was worth in any given historical period is that simple references to inflation do not take into account price inflation--or deflation.
In 1765, at the time of the Stamp Act crisis in the American colonies, specie (gold and silver coins) was hard to come by in the American colonies. In Virginia, the chronic shortage of specie meant that transactions for more than small change were most often calculated in pounds of tobacco. Tobacco was the major monoculture in the colonies, and Virginia was then the largest producer of tobacco. Peoples' salaries, for example, would often be calculated in so many hundred weight of tobacco. Patrick Henry first made his name as a lawyer by defending against a member of the established (Anglican) church who was suing for an increase in his wage--which was calculated in hundred-weights of tobacco--because of the slump in tobacco prices which had suddenly and dramatically affected the economy of the colony. (Henry lost his case, sort of--the jury awarded the preacher damages of one penny.) So, since for tax and records keeping purposes, the value of hundred-weights of tobacco would be converted to pounds, shillings and pence, based on the last official valuation of a hundred-weight of tobacco, what the real value of someone's salary or estate would be could fluctuate wildly.
Dadpad's reference to the value of consumer goods is far more reliable a method. It is not universal, however. So, for example, a man with 40 pounds sterling in income in Boston would be well off in 1765--that would support a man and wife, six children and two live-in servants. In Philadelphia (in the midst of prosperous farm regions in Delaware and Pennsylvania), it would have been worth somewhat more. But in London in 1765, 40 pounds would have bought considerably less, because of price inflation. That would be 14 shillings a week, and that's two shillings four pence in a six day work week--and would be just enough (barely) to keep a common laborer and his family. A loaf of bread such as we know in the United States was a penny loaf, and working men in Europe in the 18th century ate from three to six pounds of bread a day--they often got little else to eat other than bread, which in a complicated way explains the crisis which lead to the French Revolution, and the political crises in France in 1830 and 1848. Two shillings four pence is 28 pence--so a man had to find three to six pence for his own bread, and the price of bread, and perhaps a little butter or cheese for himself and his family out of that, along with rent. Small wonder that working class people rarely ate meat or fish.
In 1840, a common laborer (in the United States) made about 40 or 50 cents a day, and could live decently on that--except in cities such as New York or Boston, where food stuffs were expensive. They would be less expensive in cities such as Philadelphia or Chicago, near areas of large-scale food production. New York ought to have been cheaper, being near the "Garden State" of New Jersey, but there was significant price inflation there because of the large population, making it a seller's market. But on a generous basis of 50 cents a day, that's three dollars a week (in a six-day work week), which is $156 per annum. Seven thousand dollars is almost 45 times as much as a common laborer would make in a year. That is a considerable sum. If one calculates based on a minimum wage (in practical rather than legal terms) of $7.50/hour, that would be over $15,000 per annum, multiplied only by 40 makes a sum or $600,000. Roughly speaking, i would calculate, therefore, that $7000 in 1840 had roughly the buying power of a half-a-million dollars or more in today's world.
It is important to keep other things in mind as well. There was no income tax in 1840, so she would have kept almost all of her money. However, public speaking meant travel, and if she wished to lodge herself decently and feed herself well in a world of slow, smokey railways and dirt roads, and filthy, bed-bug infested inns or very expensive hotels, she would have paid a great deal more proportionately in travel expenses than would be the case today--and no one thought in terms of defraying someone's expenses when they were invited to speak somewhere. By the same token, however, with that in mind, sympathetic residents of communities in which she spoke very likely would have offered lodging and board while she was in a town or city, and would not have expected her to pay. In most cases, her travel expenses would have been largely expended on coaches, hansoms (horse-drawn taxis) and railways, which were as expensive in the terms of 1840 as are airliners and urban taxis today--and considerably less comfortable.
At home, her style of life could have been very expensive. People owned, usually, far less outerwear--changing one's linen daily was a perquisite of the wealthy--even they did not necessarily rely on a closet full of clothing for a new outfit each day. A man probably had two or three winter suits and two or three light-weight suits, and that if he were well-off. A woman might have a half-dozen dresses, if she were well-off. There were two options for clean linen daily--paying a washerwoman to come into the house each day (only the truly rich kept live-in washerwomen), or sending laundry out. Public services such as laundries were expensive, because they were labor intensive, soap and bleach were expensive, and water would be heated on the premises, using wood or coal. Eating would be expensive for the well-off, because they expected to eat three or even four times a day (the poor and working class did well to eat twice daily--upon arising and before going to bed), and they expected to eat meat at least once a day (if the working class ate any meat at all in America, they ate cheap, fatty pork). For that purpose, she likely had a cook, and the cook would want at the least one girl (scullery maid), both of whom likely were live-in employees, for whom meals and clean linen would be provided. To travel about town, she would either have kept a coach or carriage, which meant at least two live-in men, and a coach house and one or more horses, who would have to be fed and shod--or she would have had resort to expensive livery stables.
The poor and the working class rarely bathed, rarely changed their clothing (if devout, they might attempt to keep on set of good clothing--their "Sunday go to meeting" outfit), and might have only one set of bed linen (bed curtains, bed linen and blankets and "comforters" were sufficiently valuable that they are almost always listed in the wills of the well-off)--if they had any bed linen at all. Even the well-to-do would have bathed infrequently by our standards--daily bathing was a sign of true wealth, and usually was labor intensive, requiring the attendance of from one to three servants. This woman might have made really good money, but if she lived comfortably, she spent a lot of money, most of it on labor and "found" (the food given to servants and laborers) to assure her comfort.
The truly rich relied upon huge numbers of servants. In a really wealthy household in America or England, there would be a butler, who would supervise at least one footman, and usually two to six footmen. There would be a housekeeper, who would supervise at least two maids (an upstairs and a downstairs maid), and usually four to six maids, with an additional "lady's maid" for each adult woman in the house. Children would be supervised by a governess and an additional maid. The Cook ruled over a minor kingdom of her own, usually with two kitchen maids, and from one to four "girls." In addition, the Butler would supervise service at meals, for which he would enlist the services of two or more of the footmen, and two or more of the kitchen maids--they would receive relays of food from "knife boys," from one to four, who were in the charge of the senior footman. In addition to the kitchen maids and girls, there would be one or more scullery maids, girls who were "girls-of-all-work" and whose major responsibility would be dish washing and scouring posts and pans, usually assisted by the knife boys. Higher level employees--the butler, the housekeeper, the cook and the governess--would have a small room of their own. Maids and footmen would be lodged in small rooms with two or three person per room. Scullery maids and knife boys usually slept on a bench in the kitchen. The final large labor division would be for transport. This would, in a truly rich household, mean a coachman, two footman, and two to four postilions (boys who stood on a platform at the back of the coach or carriage, and jumped down to lower the steps or to run errands). They would also be lodged and fed at their employers expense--and usually were lodged in rooms above the coach house and stables. Larger estates would probably also have from one to six gardeners. All of them would be fed at a servant's table, except the knife boys and scullery maids, who were the "bottom feeders" of the household, they literally ate the table scraps from the main table and the servants table, and might have to fight for it, or have the choicer bits taken from them by the footmen and the kitchen maids.
All of those employees would entail the expense of "livery"--they would be clothed at the expense of the employer, and the butler, front of the house footmen and maids would probably have clean linen each day. This would likely mean a washerwoman and one to three girls. A household of this type could employ from a dozen to thirty or more servants.
What that $7000 was worth to that woman would have been determined by how well she lived--and each step up in ostentatious comfort would entail very large increases in expense, because all comfort in those days was labor-intensive, and included at least feeding servants, and probably clothing them as well.