I would expect all multiagency systems charge users on a per radio basis, unless they're very small. They have fixed expenses related to total size of the system (links, power, payroll, etc), so the more radios they support, the higher the costs go. The RCS board minutes are available online, and you can see their budget figures discussed there. If I recall right, it was something like 5.6 million dollars a year for their total budget.
Bear in mind San Diego county is not easy to cover with an 800 MHz trunk, with city, mountains, and desert areas, widely varying over a good size county. It takes a lot of towers and links to keep it all tied together. Expensive.
Also, charges are different (lower) for mutual aid designated radios on the system. The $75 is for a radio used for full dispatching (and thus, heavy use) on the system.