The stronger the Contract, the more often the price will be harsher. However, evidence in the story suggests for people loved by their gods like Tristan they'll get a fair or reduced price. One girl who could sense the most likely cause of her death for the next three hours only had to make tiny shrines every year or so. Contractors are not priests. Those are different. We don't know a lot about High Priests yet.
Contracts are made with Gods. Often the Contract is designed in a way to feed a God. Fortuna is the goddess of Tristan and she will make it so when he takes extra good luck, he gains in return bad luck.