I fix them all the time. I use lots of dawn dishwashing liquid to flush out all traces of gas (so it won't blow up) then I weld studs (most body shops should have a stud welder) to the tank and pull the dent out. Finish out with a little body filler and prime. Depending on how big your fist size dent is, and the color of the bike, it could run you from as little as $250 for repair and paint, up to many hundreds of dollars. Then it still needs to be sealed. Sealing a tank is labor intensive, but you could do that yourself and save some bucks.Any way to fix a fist-sized ding on the fuel tank on my Yamaha VStar 650 or will I need to replace it?
I would do like Guy suggested, and watch Ebay, sometimes you can pick up a tank pretty reasonable there.
If you decide to repair your tank, make sure you take it to someone who knows a little about painting and repairing gas tanks, its not like painting a car. Remember, gas is a solvent, and solvents can damage or remove paint. Plus, sometimes a little welding may be required, and that takes some expertise to weld on thin metal.
Good luck, let us know how it turned out for you.