What does it take/cost to get a political billboard on a highway?

[brentspeak]

I have this delusion that I would pay to post a billboard on a major highway in some swing state (Pennsylvania is closest to me) with a big picture of Bush reclining on his hammock and a message that reads: "George Bush: The anti-terrorism president takes a vacation --for the entire month of August,2001", or words to that effect.

I don't know how much these things cost, or if it's even allowed.