Or ask for a one time fee, based on the amount of money they spent. And provide an obfuscated version of the model to people.
I will buy it just like I buy other digital stuff like games or movies. Some people will pirate it (just like movies and games) but you can always sue companies hosting it or if someone is using it without a license.
Incrementally improve the model and release new versions of it. Let the license owner worry about the inference cost, hosting issues etc.