It does seem to be a social taboo to ask such a question. I think you have a very legitimate reason for wanting to know. Of course, some people can live very well on small incomes while others can't make ends meet with millions. Wages also vary depending on what part of the country you are living or even what part of a state. Example: NYC legal secretary makes about $65,000 per year, two hours away going northwest, a person with the same skills makes about $30,000. The only time I remember discussing salary with colleagues was when I worked in sales and it was posted each month who sold what and you could easily figure out how much a person made from that.
Lower wage jobs usually post the pay in want ads. The better the money the less likely you will see it advertised. Sometimes salary is adjusted based on experience. Your friend might make big bucks because of her talents and job history. A new person to the field would make much less. Salary can also be negotiated - try not to name your price, let the employer make an offer. Most people sell themselves too low.
You might want to check into sites like this one to compare:
I always liked the idea that you make yourself rich by making your wants few. You might also want to explore books like "Your Money or Your Life", I don't agree with the financial advice of the book, but the basic philosophy is a good one.