When I knew guys who actually did porn for Colt and Falcon, they explained it this way:
If you're really hot, muscled with a big dick and will go all the way, AND you have some credits to your work, you get the big bucks.
Top OR Bottom isn't as important as how much work you've done, how hot you are and if you're willing to be versatile.
A lot of the porn I've seen recently....nah, too many twinks, not enough real men. I wouldn't pay a lot of 'em $100 for the whole shot!