Not sure what size you need for your boat but I would look up Moeller Gas tanks either here at the iboats store or online to get a rough price. I bought a 19 gallon tank for under $200. I'm going to reuse the fuel line hoses since they are in pretty good condition.
I think it is $350 on moeller for a 34 gallon. I was more or less wondering what the labor costs as I have no experiance with floors and fiberglass. I would have to let someone do it who knows.