Anti-fouling bottom paint runs approx. 89.00 to 198.00 per gallon. The boat will need to be stripped with other expensive products to prepare the gel coat for adhearsion and painted with a couple of coats. Then add the labor. There are anti-fouling paints without copper.
This is not needed or recommended for fresh water boats. There are some very good fiberglass stain removers on the market. Interlux has an excellant one, if that is why you are considering bottom coating.