My buddy just bought a nice tank from ebay for $150 +$20 shipping, but I know,----they're evil. If you have access to a bead roller (though not totally necessary), you can make your own floors, that's what I did.
http://www.nationalchevy***oc.com/ http://www.chevsofthe40s.com/ range from 93 to about 120 bucks per pan at these places
What ever state you live in , they make the cheapest florpans , they're free , and found on the side of the road on steel poles , they have things wrote on them like "stop" , "speed limit 55" , etc.....
you don't want to get the cheapest. the cheapest is useless flat sheet with a few weak beads in it you see on ebay. I don't know how a person could sell that **** with a clear consience.
I just bought a '65 Mustang gas tank for mine to install in the trunk, looks real clean and was only around $150