Mine were replaced before I bought the boat so I don't have a cost estimate, but I have an idea of how they did it.
On my boat, the area aft of the salon hatch to the aft door threshold was cut out for tank removal. The hole was the width of the hatch and ran from the fwd hatch edge to the door threshold. New tanks of the same size (125 gal each) were fitted and then the flooring was replaced. My cabin floor is carpeted.