Meg Whitman proved you can't buy an election in California.
Most congresspeople spend more time raising money (banging the phones, attending fundraisers, etc) than they do on policy. You need a few million in the bank to be a "credible candidate", and lobbyists can get you a chunk of the way there.
Modern Vote Buying In The U.S., Koch , et al. 2000-
This post is dedicated to mathematics.
The fact that politicians go to great efforts to raise campaign funds proves that they think its worthwhile.