WAAS is a form of differential GPS (DGPS), nothing more, nothing less. It improves accuracy by a) proving an accurate ground based position, b) improving error checking of other satellites and c) importantly for instrument approaches, it allows triangulation from a low reference to give better vertical location. Trying to triangulate from overhead satellites results in triangles with very acute angles which has poor precision. You can have as many satellites as you want does not improve vertical accuracy. You can already buy handheld GPS units that will receive GPS, GLONASS and Galileo satellites. I think that gives access to maybe more than 60 satellites. How many do you need?
Without knowing the costs, I don't believe WAAS stations would be expensive. The money would be in the building, antennae, UPS, etc. The technology of the WAAS transmitter is cheap. It will, however, require ASA to create a bunch of new certified approaches. Maybe that is the real stumbling block?