Monthly service costs

Hello! I am new to this forum. I have been working with cams and systems of all flavors for a while. I am looking at adding security camera systems to my company lineup. I have talked to some other vendors who are not real close and they tell me that they charge a monthly or quarterly maintenance contract. It makes sense to help cover the support calls of "how do i do this", or rebooting devices. I am looking for basically another revenue stream as well as maybe warranty assistance in the event that I would need to "eat" a service call or bad component.

What do others charge for a maintenance or support package, and what does it actually cover?

thanks