Repair complexity
Replacing a fixture in place is a different job than opening a wall, rerouting a branch line, or chasing down a leak that only shows up sometimes.
Access and restoration
Getting to older pipe behind plaster, brick, tile, or a finished basement wall adds protection and patch-up work on top of the plumbing itself.
System choices
Tank size, material compatibility, valve grade, drain condition, permit requirements, and after-hours scheduling all push the number in one direction or another.
Planning note
Ask for a scope in writing, tied to your actual house. Any number given before someone’s looked is a guess, not an offer.