Look at this from Rob Neyer's new column on ESPN.com,
Let's put it this way: Mark Buehrle just signed a new three-year deal with the White Sox for roughly half the money Pettitte got ... and Buehrle's a better pitcher.
You don't believe it? Below are innings and ERA+ for both Buehrle and Pettitte
Buehrle 2001 2002 2003
Innings 221 239 230
ERA+ 140 129 108
Pettitte 2001 2002 2003
Innings 201 135 208
ERA+ 112 134 109
Buehrle's better and he'll give more innings, at least if history's any sort of guide. And yet he's not making nearly as much money as Pettitte, merely because he happens to pitch for the wrong team.