CO2 prices

Last week I went to the welding supply shop to exchange some empty CO2 tanks for full ones. I've yet to find any shop in my area willing to fill my tanks, so exchange seems my only option. I had three 20lb tanks and one 5lb tank. Oddly the price for a 5lb tank trade was $15, while the price for a 20lb trade was only $17. I guess the cost reflects labor, not the gas. I went searching for some information on the beer forums I frequent and found a post from fellow homebrewer that works in the gas industry. According to him it costs supply companies roughly $.06 per pound of CO2. Now I understand there are other costs involved (like certifying tanks, regulatory charges, etc) but that's a pretty solid markup. Maybe I'm in the wrong industry.