Is there some sort of a discount program from manufacturers or carriers in the US that allows developers get most common Android based phones for testing?
Typically thats a big no, specifically because wholesalers HAVE to make some sort of profit, so they sell their products in bulk to manufacturers for a set price. When you see the cost of a phone a certain percentage of that cost goes to the copmany that made the phone and the other percentage goes to the service provider (att, t-mobile, vzw, etc).
The only way I see this possible is if you were to become an employee in the test market of a phone company and even then your chances are slim that you would be able to actually buy the product.
I said Google because Google is the one who really cares about the whole apps thing. Right now Android phones are doing just fine with our "sub-par" app market so the manufacturers don't care too much.