Eli isn't overpaid and the reason is simple, there aren't enough guys to do that job. Eli is a good enough QB to win a Super Bowl, it's happened twice. The Giants don't want to have to go back to the Dave Brown and Danny Cannell years, so when you have a QB you can win with you keep him and pay him his money. The going rate for QB's is around 20 million a year. Eli might not be Romo, but he's solid enough to win with under the right conditions, and he's a genuine iron man, he has something like 160 starts in a row - I don't think he's ever missed a game due to injury in his career.
All that said, I want to make the Giants regret it before the ink is even dry. Harassing Eli into about 6 interceptions on Sunday night should do it.