I don't think you have much choice but to make arrangements to shut the building water down if you want to replace that valve.
Pricing is really tough as all areas labor rates, building permits, etc. are different. Assuming this needs a permit and will be inspected, that by itself could add hundreds. If in the process of removing the old one, the galvanized piping is found to be in too bad of a shape, it could cost LOTS more, but often, the condo association owns the 'infrastructure', and would have to pony up that repair cost.
Galvanized piping is a disaster waiting to happen. Touching it is fraught with potential pitfalls.