The Internet Archive's Brewster Kahle has done the math on building a data-center that could hold all of America's voice-calls, and concluded that this it wouldn't quite fit within the $20M price-tag reported for Prism, though it's not far off.
These estimates show only $27M in capital cost, and $2M in electricity and take less than 5,000 square feet of space to store and process all US phonecalls made in a year. The NSA seems to be spending $1.7 billion on a 100k square foot datacenter that could easily handle this and much much more. Therefore, money and technology would not hold back such a project– it would be held back if someone did not have the opportunity or will.
Here's a shared spreadsheet with Kahle's calculations.
I write books. My latest is a YA science fiction novel called Homeland (it's the sequel to Little Brother). More books: Rapture of the Nerds (a novel, with Charlie Stross); With a Little Help (short stories); and The Great Big Beautiful Tomorrow (novella and nonfic). I speak all over the place and I tweet and tumble, too.