There are some rules in the DMGII, if you want them, around p154.

“Typically, an adventurer’s daily fee in gold pieces equals her character level squared, plus a split of any treasure gained – usually a half share of the total loot. However, she charges ten times the normal asking price and demands a full share of treasure if her character level is equal to or greater than the average party character level.”

(By "adventurer" here they mean "NPC with PC levels", but go read the whole thing if it sounds like something you want to do, there's more in the book.)