Question about map development

Hey,

I am looking to make a Prison RP server with a custom map.

I want to take this map https://steamcommunity.com/sharedfiles/filedetails/?id=731181567 and add a city that can be accessed via a boat or swimming

(Kind of like alcatraz) except I just want it to be a small one where prisoners can escape to and have police patrol and a few citizens. Nothing big.

I get that most people like to get payed by the hour but I don't know how much to charge. Anyone have any idea?

Thanks for reading.