Contractual Magic - for an unnamed vote-created setting

[Gnorman]

So I find this a very interesting idea, but I have a question:

Is the goal to contract (1) with entities, or (2) for services?

For example, if I want to become more resistant to fire, do I call up one of a wide variety of entities that might offer such a boon, be they devil or efreet or angel? Or do I call up an efreet first, and then negotiate for fire resistance as part of a larger package deal?

Some spitballing below:

Entity: Whom (or what) you're contracting with. Perhaps there is a token price just to bring them to the table, much like a spell component. To entice the imp to even consider your offer, you've got to bring some rancid meat for him to snack on.
Contracting DC: The DC to make the contract, obviously. What is the DC based on? An Intelligence check? A Charisma check? Diplomacy? Bluff? What kind of modifiers can you get? What are the benefits/penalties, if any, for spectacular success or tragic failure? How many contracts can you have at any one time? Are some contracts mutually exclusive?
Benefit: What you get when you successfully make the contract. I like the idea of using cleric domains here - maybe you get a "contract power" no matter what your contracting result was, and certain spell-like abilities if you succeed. For example, you make a contract with an efreet for his services. The DC is 25. Even if you fail the DC, you get Fire Resistance 10 for the day. If you meet the DC, you get to use Fireball 1/day as a spell-like ability. If you beat the DC by ten or more, you get to use it 3/day. Your skill at contracting allows you to negotiate better deals for the price.
Price: What you have to pay for the benefit. I think services are better than items or gold (more interesting effects), but you could easily throw a wide range of payments here. An efreet might exact as payment: "Set fire to an inhabited structure today." Negative payments could be possible too - a dryad might exact as payment: "Do no harm to any plants today." Actually, the latter might be more interesting, as the efreet's requirement affords a lot of latitude in putting off the payment until the end of the day.
Breach: What happens if you don't pay the price. Could be as simple as "the entity refuses to contract with you again for a week," or as dire as "the universe strikes you down for refusing to conform to its laws, take 4d6 CON damage."