You shouldn't overpay for ANYONE for that matter. Look at Shaun Alexander. He hasn't been the same since signing that big contract two years ago.
Alexander's problem isn't that he signed a big contract. It's that he's worn out as a running back.
Ever heard of "the curse of 370"? Basically, if a running back has too many runs in a season, he's going to fall apart afterwards. Statistically this massive drop-off in performance happens around 370. Alexander had 370 runs in 2005 and hasn't been himself since. He's also getting older, which isn't exactly conductive to a good season either, although Curtis Martin showed that it's not impossible to pull off.
By the way, "the curse of 370" actually predates Alexander's 370 run season.
When Larry Johnson was run into the ground with an NFL record 416 runs in 2006, there was never any question that he wouldn't be the same again. Sheer stupidity by Herm Edwards.
Ask Ricky what he thought of being run close to 400 times two seasons in a row by Wannestedt? After the first one, he had a career-low 3.5 yards per carry in the next. After that he quit, which (while done at an inexcusable time) probably saved him as a player, allowing him a moderately successful 2005.
With regards to contracts:
Only an idiot would sign a running back who has been run into the ground to a huge contract.