Buying a 6.8kg outright = 2 years rental charges. BOC, Supagas, etc refills are slightly cheaper than getting your own filled/swap at the LHBS but not paying rent. I own my CO2 cylinder and rent a Cellamix 40 nitrogen mix from BOC for stout.
With the bigger 6 - 6.8kg (depending on Kegking mykegsonlegs) you should only need a couple fills a year plus get more gas for your money when refilling/swap compared to a 2.4kg bottle.
2kg of CO2 ~ 1000L gas so carbonating at 2.5 vol means that you should be able to gas and serve 400L beer (20kegs) with 4kg gas (I could be wrong on this calc - couldn't be bothered checking the numbers - you know how much gas you use).
Consider the cost over 10 years (without inflation, cpi bs, etc)
rental Supagas quoted above $14.95/month for 10kg bottle = $179/year = $1790 over 10 years
refills - say you drink a lot and need 3 fills/year @ $38.60/fill = $115.50 x 10 years = $1150
total cost over 10 years = $2940
buy a 6.8kg $349 (at my LHBS)
refills $54.20 for 6.8kg so its 4.4 fills/year (30kg) = $239/year x 10 = $2390
total over 10 years = $2739
Saving buying your own $200
If you like your my mate and I who only need 1 or 2 6kg cylinder fills a year then the savings even are better
rental fills 1.5/year = $57.9 = $579 over 10 years + $1790 = $2369
own fills 2/year = 108.40 = $1084 over 10 years + $349 = $1433
saving of $963 :beerbang: