Good luck determining the "quality" you have been promised, unless you are in to colorimeters, light sources, reflectivity and such. Or the painter has specified and AGREED to some number of runs, or XX sq inches of defects, or 12 bugs or dust hits. Simply removing and replacing every piece of deck hardware, window frame and such can add $XXXX to a job. I would guess that that job on your 18' would add several hundred $$ to the job, assuming none required added labor to remove and replace. Each item then has to have the mounting points cleaned, faired and sealed. Cracks in the gelcoat as well.
There is just so much subjective and "art" that goes in to a good paint job and a lot of it, you will not see for years.
I have (on a very few occasions) provided a statement that the boat will be "run free", no orange peel, less than XX sq inches of dull finish, etc AND I WILL repair at MY expense AND - ASSUMING that the customer pays for my prep. If it is merely a clean and spray type job desired by a customer AND he wants perfect, then all bets are off. A good paint job is about 75-80% prep work, and almost always beyond the "normal" work of a yard/shop/painter and what the customer is willing to pay for.
If you consider the price to be "high side" - it probably is, and unless the painter is a perfectionist, you know his work, and have seen more than one or two reference boats he has painted....you are wasting your money, as there is no easy way for the two of you to agree on "quality"
Good luck, as the only contract I have ever seen, is usually an estimate that states so many $$ per hour, material costs and a ball park for XX hours. And then a few paragraphs/clauses on the back as to how those numbers may sky high, and you will still pay.
If you are that worried about it, trying to give him money, you need to find another painter/shop.