That's a simple question with a complex answer. The short response is "It depends".
Did all the agencies on the system pay into the construction of the system?
Is the system owned by a single agency, multiple agencies, a consortium/district, or a private entity?
Is the system managed by a single agency, but sites were built by individual jurisdictions?
I'm sure that there are many, many other questions that are involved, but here's an example from a private TRS:
The Radio Shop sells Company X a number of radios. The Shop programs their radios with the frequencies used by the system that the Radio Shop has built. The Radio Shop then allocates as many talkgroups as Company X requires. The Radio Shop then charges Company X a fee for the use of the radio system. This fee could be a monthly, quarterly, semi-annual, or annual payment. The payment could be "per radio" on the system or it could be "per radio * per talkgroup". It also could be per radio with a per minute airtime charge.
If Company X wants additional features, such as Telephone Interconnect (phone patch), AVL (FleetStat (tm)), "Private Call" (unit to unit calling), etc. then those could incur additional charges. If the Radio Shop has a "wide area" system with multiple systems that are linked together, and Company X wants that wide area coverage, then the Radio Shop could also charge for that, as it would tie up multiple resources across the system.
For example, look at
TeamTalk Networks for an example of a wide area private LMR network.