The cost will vary widely, depending on what the nature of the problem that one is trying to solve. Make/model/year, and what's the issue that prompts wanting an overhaul? Some things are very easy to solve without tearing down the whole engine. A complete overhaul renewing ALL wear parts will easily get into the multiple thousands if you are paying someone else to do it.
I do mine myself, sending out only machine work and cylinder head work (which I don't have the tools for).