Hardware grade and type
A basic residential knob or deadbolt costs far less than a Grade 1 commercial lever, a mortise lock, or a smart lock. Higher grades last longer under heavy use and resist forced entry better, which matters most on exterior and commercial doors.
Door and frame condition
Rotted frames, misaligned strikes, or doors with old non-standard bore holes may need extra work before new hardware will operate reliably. A good locksmith points this out before starting, not halfway through.
Number of locks and keying
Replacing several locks in one visit usually lowers the cost per lock. Having them keyed alike, so one key opens every door, adds convenience with little extra cost.
Rekeying as an alternative
If your existing locks are in good shape and you only need old keys to stop working, rekeying can cost significantly less than replacement.
How to get an accurate estimate
Send photos of the door, the existing lock, and its edge, along with your address. Confirm what the estimate includes, and approve the price before any work begins.