Comment by blintz

The simplest approach would be to just charge people per-query (or charge in levels, depending on the number of queries). This could be done in the standard way (you have to log in, pay the site, and then it gives you an API key or just session token, and logs how many queries you make). I think you can avoid having to use a ZKP this way, since that will make things much more complicated and possibly costly.