Relatively speaking, not expensive. There are half a dozen (or so) ribs, some angle that gets cut into stiffeners, a rear baffle, a fuel drain, a fuel cap, some aluminum tubing, a chunk of heavy angle, and a couple of fittings.
I'd guess $250 plus a pint of proseal and whatever shop supplies you'd need. Worst case, shouldn't be over $500.
Just build a pair, just over $500 each, depends on options, new locking caps? new fuel senders?, pickup tubes?, fittings?, do you need to buy rivets? sealed pull rivets for the rear baffle? Can you use parts from your old tanks? I changed all of the hardware except for the fuel senders, they checked out fine, I don't rely on them anyway. I'm liberal with Proseal, but no leaks. Plus depending on your experience 25 to 40 hours of labor.
Have fun!!
PS the factory does not have a list of parts for a fuel tank, you need to provide all the part numbers.