I don't believe the 2Billion dollar amount for a second. 2 Billion seems like a very VERY large number. Now lets do some simple math. Divide that by 50 and that is 40million available for each state. You might be able to protect Wyoming for 40 Million, but I doubt you could protect Alaska (construction costs are too high) and I doubt you could even begin to protect a city like San Jose, New York, Dallas, or Chicago for 40 Million (not even to mention the rest of those states).
The other interesting thing to think is protect against what... Yes, you could probably protect against a low yield EMP produced by a solar flare - however, there will be no protecting against a high yield nuke detonated over an area (where the EMP is the least of your worries). EMP is only one of the things that would need to be protected against - there is blast damage, fallout, fuel supplies... Yeah - if a high yield bomb goes off over the US we are all screwed, whether we spend 0 dollars, or 100Billion.