Gas leaks when filling the tank

Perhaps it is just the filler neck leaking, the pipe between gas cap and tank. Not sure about cost. Fuel tank might have to be taken down to remove filler neck.

It could be as simple as tightening up a clamp.

The Medic

If you are in a salt state my bet is that the vapor vent tube has rusted through at the bottom. Salt and dirt collect there and the metal rusts. The two circled areas are usually where they leak.