A) Give them access to the database via a simple HTTP-based API. Let them run searches, or whatever they need against it. Restrict their queries per second and

A) Give them access to the database via a simple HTTP-based API. Let them run searches, or whatever they need against it. Restrict their queries per second and per day and limit how many results can be returned. Make sure you can track their usage and cut them off if they're doing anything shady. Have them sign a service contract. Charge them a setup fee and a monthly fee that's based on the number of queries they need to do. B) Another option is to give them the entire database, and provide them with regular updates. They pay $100k for the database, and $5k every month for the latest version. This doesn't protect you that well, but if they need the updates it can be a good way to keep them honest.