Wired had some time ago an article about one NSA data center [1]. That article had some additional numbers on the scale of the surveillance efforts. Specifically it did note that the specific site will have a 65 MW power station, which one can compare to some supercomputers. For comparision RIKENs K Computer demands roughly 12 MW of power. [2] ( One can also try to estimate similar numbers from the size of the facility and from the building costs. In all cases one gets a similar factor of a few above modern supercomputers.)

[1] https://news.ycombinator.com/item?id=3711603

[2] http://top500.org/list/2012/11/