You're right of course, but the difference is Android is free (or almost so), as is Symbian, whereas MS charges a per-unit license, so at lower price points that would eat into their margin too much. That's the only reason I don't see WP7 going into cheaper phones tbh, not its minimum specs (the way microprocessors are going we're going to have SoCs as fast as a Snapdragon in £50 dumbphones in a couple of years).
Microsoft won't stick to a strict per-unit licensing model. They don't in the PC world, why would they in the mobile world?
I'd expect anyone selling tens of millions of Window Phone devices a quarter to get a very healthy discount.