I have been into many forums and found this "kegging" using CO2 to be a very popular form of carbonating beer. Of course the cost factor has put many off, including myself.
Searching through the internet I found this really smart idea which I think I may try out: http://inventionsthatwork.com/carbonator.htm
It uses baking soda (sodium bicarbonate) + citric acid to cause a reaction which naturally produces CO2 (like alka-seltzer tablets). If you lookm at the picture of the carbonator, it is quite simple for a handy person to drill a hole into a PET bottle cap, insert a barb nipple ob both caps, join with a short hose and voila! your endless supply of CO2 for pennies. I will try it out this weekend to see if it works.
If any of you try it out first, do let's share the outcome.
Cheers
homemade CO2 charger for kegging?