$700?! I hope that includes installation and a full tank of gas!
I checked at
Discounted Nissan Parts and they have a genuine Nissan fuel tank for a 96 Pathfinder for $291.23 (MSRP list is $397.85). I wonder if your local dealer is quoting a fuel tank with a fuel pump, perhaps?
Another option would be to have your tank repaired. Re-Nu is a process where they take your old tank, cut out the rust, sand blast it, weld in new metal and coat it and back it up with a Lifetime Warranty. Usually you find this system in places that repair radiators, so check your yellow pages. If you have no luck in your area, try this link:
Moyer Fuel Tank Renu - antique gas tank repair restoration, classic car gas tanks GASTANK
Another option would be to get a used tank from a salvage yard.