I bought my first CO₂ refill today. I was comparing the receipt to the one I got when I filled my take the last (first) time over a year ago. No change in price! It cost me $11.23 for 5 pounds of CO₂ on 9/21/2010 and it cost me the same thing today.
Since that time, I’ve done 10 batches of beer, so that is my first data point on how long a tank lasts. I probably could have got a couple more out of it. It was not absolutely shot. I did mess around a little to learn how it worked at first, too. I made some carbonated water. I also transferred some sanitizer between kegs and carboys. Maybe I could have gotten as many as 15 batches.
That would be 75 gallons of beer carbonated with 5 pounds of CO₂ at $11.23. That is 800 beers at 1.4¢ per beer.
That is not too shabby.