I can't imagine it would be more than a couple hundred bucks assuming they can find an accessible point for it. Mine is in an unfinished room at the corner of the basement where the main line comes in and replacing it took the plumber life twenty minutes.
You can ask the plumber about coordinating with the city for shutoff. Maybe they'd know a guy?
But definitely don't mess with it if. In my case I couldn't find my external shutoff when I needed that internal one replaced so I had the city come locate it for me. They uncovered the plate, tested the valve and it broke immediately. Fixing it was a major excavation. The city guy told me how lucky I was that I hadn't found it and tested it myself.