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Biggest part of the expense in replacing the tank is the labor - to make the new tank and to remove the old tank/install the new one. If your deck needs to be re-cored, I would attempt that using Divinycell or another lightweight core material instead of plywood, and install aluminum backing plates where the console and leaning post will be attached. Once re-installed, you will have a solid deck that will last for 30-40 years. It is a big job, which is why they quoted you $3k to replace it I would guess that they will add a clause that allows them to raise the scope of work/dollar amount once they get in there....and they will exercise that clause.

All told, that is a very reasonable quote, but it won't include re-coreing the deck if needed.