Cost and value don't apply here. Honestly, I think they've provided very little value. The fundamental truth is that terrorists are still a tiny minority and while they may have disrupted an attack or even a few, they didn't stop Boston and there simply haven't been hundreds of legitimate plans.
That leaves insane costs with little to no intentional value and tremendous risk for corruption. Hard to shutdown but not impossible.
Their are a large number of secret 'fiefdoms' that have been created since 9/11. The people in those positions of power do not want to see those positions threatened. Therefore, when you have such power and knowledge about everyone, you can also threaten anyone, since everyone has something to hide. Including senators, judges and presidents.
That leaves insane costs with little to no intentional value and tremendous risk for corruption. Hard to shutdown but not impossible.