I have had a tank filled at a beer distributor in PA. That was like $5 but they don't actually fill it with liquid, just pressurize it to 800psi as a gas. A real fill usually means swapping the cylinder out and runs 15-20 dollars for a 5lb around the US at a welding shop, fire extinguisher, or simply gas supply place.
If you go up to a 10 lb it might be 20-25$. Point is you get ripped off by using a smaller tank!