Here's some ideas I had.

  1. Use the cleric domains to make a list of general things contracts can be formed with.
  2. Contracts have a base price at a base DC, the price will go up or down with DC.
  3. As Vauron said, prices can be services (i.e. good wants good done)
  4. Other other possible things could range from something simple like not eating for a day to something really big like giving them your soul.
  5. Some contracts will have prerequisites (air doesn't like earth, vice versa)
  6. At some Level Contract makers can make contracts with regular people (small bonuses with a geas-like effect)
  7. There should be lots of modifiers to contract making. Such as a penalty for being under duress. (Your soul for an instant teleport sounds better when you're about to get eaten by a dragon.)