I think FDA rules explicitly prohibit paying blood donors in the US. Ostensibly because if you do, the donation centers fill up with junkies who'll lie about not having hepatitis so they can get paid, or steal IDs so they can go twice a week until they die of anemia, raising the costs of safety testing and generally being injurious to public health. Of course, everyone else involved in the process gets paid, just not the donors -- quite dearly, as you'll learn if you're ever on the receiving end of a transfusion.
Plasma is paid because it falls under a different regulation and their research and industrial customers don't care that the plasma came from a crack whore.