The cornie I got was converted to use the S30 Hambleton Bard CO2, which lasts a good long time if you're using it for serving rather than force carbonating. Local shop charges about £4.50 to swap the empty for a charged one, so it doesn't break the bank.
I don't have facts and figures because I just kegged my first brew in it.
The sparklet (soda stream) ones are not cost effective, since they are one shot jobs with no way to reseal or control the CO2.