Since prop players are mandated to play and generate action, and the affiliate is paid a piece of the rake, it ends up working out nicely for the affiliate.
Many programs will set a system up whereby the prop players must play a certain number of hours or hands per hour, at certain limits to qualify.
Sometimes they cap the affiliate revenue as the prop program can be seen as artificial action. However, most often, the affiliate pays the prop players out of the rake they generate for the affiliate.
Basically, the affiliate says to a group of players, I will pay you $6/hr (insert $ here) to play for me. Then he contacts a poker room that needs action and makes a deal that he will supply x number of real money players who will play y hours a day.
I hope that makes some sense.