Im looking to get other peoples inputs on CO2 fills / swaps. I have only dealt with AirGas so far and it was $21 for a 5# swap. I have also heard that Way to Grow will fill larger tanks for around $8.
Best thing to do is to call around. Colorado Compressed Gasses are really cool people and 9 times out of 10 will beat air gas (which I will avoid at all cost). General air and US Welding is a few more to check out.