When I was in college, a guy used to pay me to suck my dick. He was not bad-looking, in his early 20's, and rich. The first time we hooked up, he took me out for a sub and a beer after we'd been working out at the gym. I got more than a little buzzed. We went back to his place, got naked, and he sucked my cock til I came. I was getting ready to reciprocate (I think reciprocation is very important) and he said "Nah, next time." So we watched a movie or something that was on tv. A few hours later, I had to get dressed to make my next class on time. I found a crisp $20 bill in the pocket of my jeans. It was the last week of the month, I was broke, and had been eating peanut butter spread atop stale saltines.
He introduced me to some of his grad school friends, too. They paid better than he did -- usually $50 to blow me. Getting my dick sucked was a lot less work than anything else I did at college -- and paid a lot better, too.
I don't know why any of these guys paid for sex. They were good-looking, disease free, and generally had really good weed to smoke. I would have done any of them for free.