How much does it cost to fix/replace the gas gauge to a 1977 Ford F100 Custom pickup truck?

Project vehicle. Just looking for an estimate, because that's GOT TO BE the first thing that gets fixed.

Could be a real cheap fix, it just might be a bad ground

The gas gauge is very simple on older vehicles. There's power to the gauge, then a wire goes to the sender in the tank. Does the gauge move when you turn the key on? If so, then likely the wire to the tank has been grounded. That is easy to fix. There are only 4 things that go wrong you need
1- power to the gauge
2-working gauge
3-wire to the sender intact
4-sender working.
http://www.ford-trucks.com/forums/1164132-fuel-gauge-troubleshooting.html