I use the gas to force carbonate, and also to help with cleaning. I pressurize kegs to push cleaning fluid through both the keg dip tubes, and also to push cleaning fluid through the beer tap lines on the fridge. I have occasionally had leaks, and this is included in my costs...
Since October 2007, I average 255 days between refills, on a 5kg keg. (six refills) I've had two times only 150 days or so, both the first fillup (finding the initial leaks) and after moving house (finding the leaks after breaking the whole thing down and building it up again)
That's an average cost per year, with all fillups at http://www.kolsyra.is/" onclick="window.open(this.href);return false; of 5028 ISK / year. Their prices have risen at ~7.5% per year, which is pretty much inline with my memories of inflation over the last 4 years.
Ahh, but how much beer is that?! My notes tell me 48 batches that went into kegs, or, 439kr/keg. YMMV

If anyone's _realllly_ curious, I can put up the spreadsheet for this. (Why did I even do this? Well, I was waiting for the wort to cool down)

