Repair complexity
Swapping a fixture in place is a different job from opening a wall, rerouting a branch, or chasing an intermittent leak.
Access and restoration
Older plaster, tile, masonry, and finished basements near the water table all add protection and patch-up steps to the plumbing work itself.
System choices
Tank size, material match, valve grade, drain condition, permits, and after-hours timing all push the estimate one way or another.
Planning note
Get a scope in writing, specific to your house. Any number before an actual look is a guess, not an offer.