Cost = time + materials
time:
Detective work to find cause of leak
Repair - actual work, including any tear-out to get to source of problem for repair procedure
Refinish/remodel - to restore appearance to original.
materials:
Actual materials used plus consumables used in repair operation
Some repairmen only do the investigation/repair and leave the restoration of appearance for other craftsmen. Check when you are contracting with them. A plumber may be great on pipes, but not be able to handle drywall repair at all.
Larger companies generally can cover more aspects of the problem, but you have to make sure that there is an understanding between you on what will be covered. Don't assume anything.
A set price on something like this should generate suspicion. Either it has enough profit built in to cover any possibility and it is an inflated price, or else it is too low and they may wind up losing money and abandon the job part way through or assign it a low priority.
(edit)
I see that the two answers that popped in while I was writing this assumed you are doing the work yourself, and that you know what and where the problem is and what materials you are repairing.
It is still time & materials; your time, and your out-of-pocket for materials. What is your time worth?
Do you know precisely where the leak is and what is causing it? Track it down. Iron pipe? Copper? Polyurethane? At valve or fitting, or in a run of the pipe? Supply pipe or drain?
Once started, divide the job into different aspects. I consider each aspect separately. If you can fix it with two trips to the hardware store, that's a good job. Three trips is an average job. More than three; you're not planning well enough before going.
Patience and a checkbook and a good materials supplier are essentials of fixing it yourself.