I started out on CO2 (3 -5 $'s a fill depending on where I went) and then I decided to make the switch so I kept watching Ebay for some used HPA tanks..the tanks alone were pricy but I was able to pick up two package deals from ebay (included a marker, an hpa tank, a remote line, and a pod pack - I picked up two of these, 1 with a 98 and 1 with an A5). I was then able to remove the peices from the packages that I wanted (I kept the A5 and both hpa tanks and 1 remote line) and resold the rest and I was able to just about break even on the swap. So if you watch ebay you maybe able to score a used tank as part of a set for a great price and then resell what you don't want/need. The only real investment was the scuba tank (75$ from craigslist and it has a current hydro til 2011) and the yoke (30$ new on ebay) that allows me to fill the HPA tanks. This is great b/c my son and I both play every weekend so we're going through at least 2 tanks a week and when we were on CO2 that was really adding up quickly. but now it doesn't matter how much air we use b/c the fill station is in the back of the Jeep : )

AND I can get my scuba tank filled for 6$ so each bottle is costing us pennies to fill.. a great deal if you can afford the start up cost (in my case it was a little over 100$).