I have a hard time Glenn Greenwald would stake his reputation on this without having more concrete proof. I think that there is still a lot more information to be let out. They are slowly leaking more and more every day so that the story stays in the news. It's a clever tactic. I'm curious why Greenwald isn't going to release the "highly detailed technical materials on how they eavesdrop" that he claimed he had on Twitter. [1] Perhaps it would give up his source? Or maybe it just contains too much national security information or whatever? I'm not sure.
[1] https://twitter.com/ggreenwald/status/343470800784982016
Ninja edit: Also as another commenter pointed out, that $20-million figure could be for just one specific part of the program or what have you.