I think $6000 dollars is about what you would pay for new tanks, valves and fuel lines.
NickF. has a great write up on his replacement of the starboard tank, I opted for 2 smaller plastic tanks. If you were to pay a yard it wouldn't surprise me at $20,000.
If I were to have to consider it again I should have cut access ports to inspect the inside of the tank to see if repairing in place or epoxy coating the inside could be considered. Nothing lost if you have to cut them out anyway.
These below are 2 of my threads on the same project.