Most ISPs charge between $10 and $15 monthly for equipment -- you should be able to see the fee on your ISP's broadband ...

Data Over Cable Service Interface Specification (DOCSIS) is an international telecommunications standard that permits the addition of high-bandwidth data transfer to an existing cable TV (CATV) system ...

but DOCSIS 3.1 modems are typically more expensive. Other things to consider are the modem's speed limits -- make sure they're equal to or above your internet plan -- and the number of Ethernet ports.