@JLNobody,
Cyracuz has answered your question. The prospect of plunder was what motivated the private soldier. It was very real, too. The feudal levy was never very reliable. You didn't know if your feudatories would actually show up, or how many people they would bring, or even if they would fight for you. At the battle of Bosworth, one of Richard III's supporters, Lord Stanley, held back to see which side seemed likely to win. His men approved because they felt that he would not risk their lives needlessly. The night before the battle, John Howard, Duke of Norfolk had found a note pinned to his tent which read: "Jack of Norfolk, be not too bold, For Dickon, thy master, is bought and sold." When Norfolk was killed, and the issue hung in the balance, Stanley threw in with Henry Tudor, Richard III lost and was himself killed.
But other kings had not been so foolish. The two successful kings in the Hundred Years War, which preceded the Wars of the Roses which ended with the death of Richard III had been Edward III and Henry V. Both of them relied upon the mercenary interests of their commanders and private soldiers. A captain, meaning just a leader of men, would contract to deliver a certain number of men-at-arms and a certain number of archers for a specified payment. An official of the treasury would sign the contract on behalf of the king, then cut it in half, along a wandering, jagged line, giving one half to the captain and keeping the other. At the end of the campaigning season, the captain, or his designate if he had died, would present that half of the contract, and would be paid, less any monies which had been advanced.
But that wasn't going to make anyone rich, and the captains had to pay their own men, so their was a real incentive to fight well, to be reliable. In 1346, Edward landed in France with a fairly large army, although not nearly as large as the French feudal levy. But the French dicked around, and it took forever to get things organized. In the mean time, Edward was not idle. He marched to Caen, the home of the Conqueror (the English archers and men-at-arms hated William the Conqueror and were eager to take Caen), which he intended to besiege. Caen was then the second largest city in France, and rich beyond the wildest hopes of the English soldiers in the army. Edward had had his troops spread out in a wide arc to make his army look as imposing as possible, probably in the hope of getting the defenders to agree to terms. As the troops approached the city (they really,
really wanted to take the city), they saw people on the city walls taking down their banners, and abandoning the walls. They hacked their way into one of the city gates, and realized that the defenders--the citizens of a city defended the city walls, while the nobility and their men-at-arms defended the citadel--were abandoning the city. They moved around to the south and saw the defenders crossing a bridge. The wealth of the city, all the great houses, the warehouses, the rich shops, were all on the Ile St. Jean, which was surrounded by two rivers, but had no city walls. Suffering great initial casualties, the archers and the Welsh lancers, who not only had no orders but who were defying orders to fall back, fought their way across the bridge and waded the river, breaking into this rich, rich city. Those who survived gained wealth unimaginable to them. (French merchants came along in the days after the Ile St. Jean fell, and bought silks, brocades, silverware and plate, and other bulky items at bargain basement prices, but the private soldiers preferred to have coins in their pockets to attempting to carry huge bundles of goods on campaign.) Although by contract with the king, the captains had to pay their men at fixed rates, this was what the private soldier dreamed of, and Caen brought wealth to match that of fables.
But for the captains themselves, although they profited as the private soldiers did, ransoms were their goal. Later the summer, they were trapped by the French army (the feudal levy) on a long ridge north of the River Somme in Picardy, near the village of Wadicourt, which is what they called the battle. (The main fighting was on the English right, at the south end of the line, in front of the forest of Crécy, which is what historians call the battle.) With that wonderful, stupid élan of the French knights, they attacked the English line late in the day, and were cut down in their thousands by the English archers, or the English men-at-arms when they finally reached the English line, badly disorganized. Mounted knights charging knee-to-knee are almost unstoppable. A mounted man who is not in a tightly packed formation is doomed. The Bishop of Durham kept pleading with the King to be allowed to join the battle, and when Edward finally released him, he charged into the flank of the French attack, swinging his heavy mace right and left (Bishops didn't use swords, because they were forbidden by the church to do so--maces were allowed on a technicality because they aren't mentioned in scripture). But he soon discovered that it was only him and his immediate bodyguard--his men-at-arms were plundering the French dead already on the ground, and looking for hostages. It was late enough in the day that the good Bishop was not endangered though.
And that was what the great captains were looking for--hostages to ransom. Later in the year, the Scots who had been convinced to invade England by their French allies, were routed at the battle of Neville's Cross near Durham (the good Bishop missed a wonderful opportunity, upon which the Archbishop of York capitalized). King David the Bruce was captured in that battle. His ransom was 30,000 pounds sterling. To put that in perspective, a laborer earned one penny a day--and there's 240 pennies in a pound. That ransom was the equivalent of a year's wages for 20,000 laborer's. King David was captured by a man-at-arms, and he popped him in the mouth with his mailed fist. It knocked out his front teeth, but he man didn't care. He "sold the King on," meaning he took a payment of 3500 pounds now rather than waiting for years to claim the 30,000 pounds. Everybody--who was not Scots--was happy. The ransoms from Crécy, and ten years later from Poitiers, when the King of France and his eldest son were captured, bankrupted France for several generations, and made King Edward and his nobles, as well as even humble English archers, very, very rich men.
In the 15th, 16th and early 17th centuries, the Spanish had the most feared army in Europe. They never even bothered to pay their soldiers. They just turned them loose in the nearest city, and let them "pay" themselves.