Something that I think would fit with contractual magic would be the idea of the geis. It'd be an agreement that, if broken, brings ruin upon the one who broke the oath. Meanwhile, keeping to a geis gives you power.

One way of paying contracts should be through service. For instance, Apollo will agree to ensure your rival develops a nasty case of consumption, but you must agree to clear out the den of yeth hounds that have been praying on the herds of a nearby shepherd.

While I have no problem with evil offering easy deals or ones that can be made quickly, the vile should not be the only ones to offer power. It should be entirely possible to contract with a hound archon for the ability to speak in tongues or a water elemental to gain the ability to breath water. Perhaps the darker powers demand less early on, in an effort to make the greater prices seem like just another small step compared to what has already been paid.