My experience with CO2 refills is that it does not seem to matter how big the tank you want to fill. The price to fill my 10 lb tank is within 10% of the price of filling a 5 lb. If you have a welding gas supplier in your town, drop by and talk to them. I ended up getting my tank from the one in my town at a price that would be very competitive with anything I could find on E-bay. I had looked there first and wanted to make sure I would be able to refill so I went to the welding gas shop. After talking it over with them, they sold me exactly what I needed at a very good price. It seems that a welder won't put up with the price gouging you get at the LFS so their prices are quite good.