For that amount, they should be saving in the order of 40,000 people's lives from terrorism a year to justify the spending.
(I've plucked the intervention per life-year saved from here: http://www.econ.ucsb.edu/~tedb/Courses/UCSBpf/readings/inter...
and assumed each life-save has 50 potential years of their life left)
If the NSA had a hypothetical program that didn't infringe on anyone's rights (likely impossible, but bear with me), but did consistently stop terrorist plots when they arose, even if all of the successful plots may kill a few dozen or a hundred people in total, most people would agree that spending $80 billion on that program is worth it due to the lives saved.
I don't personally think that $80 billion being spent to protect lives is wasteful, generally speaking. I do see how it is wasteful in this situation, because it has very questionable results, and when the collateral damage of the plan is so incredibly great.