Yikes bud, probably not a real pretty number. Anything I said would just be a jab at it. Also, what HP are we talking here?
I know if I had to do it, it would take 1-2 hours to lift the fully dressed powerhead off the block, an hour or two to clean the old gasket off adapter plate and powerhead, and another hour to drop the powerhead and bolt her back down. Gasket probably $30-40.
Most shops charge $85 an hour so you can imagine that will get pretty pricey.
Too bad you don't live around me, I would do a 1/2 day project like that for around $200-$250 plus parts.
This is all assuming that I am even correct on your problem (which I suspect I am.)