It is simply a matter of how much gas you will be using and what your nearest retailer charges for filling up a CO2 bottle.
I don't use my kegging setup all that much, however I have re-filled a 6KG once and I am part way through it now.
For me it would have been cheaper to buy a refillable bottle rather than disposable bottles (which I didn't see at the time I bought mine).
The other consideration is if you want / need to sell the CO2 bottle in the future. It is more likely you will get more than 50% of your money back for a refillable bottle, while you are likely to get very little for a partially used disposable bottle. Another bonus is that by going for a refillable bottle, you won't be creating more landfill.
If you look at the Keg King site, 2.6KG of a refill is $25 and the bottle with 2.6KG of gas is $199.
There is a full 2.6KG bottle on Ebay for $149+18 delivery.
Using the Ebay bottle as a starting price the costs are as follows:
2.6KG $177
5.2KG $192
7.8KG $217
10.4KG $242
If you don't mind a bigger bottle, then a 6KG bottle would be another good option, especially if you carbonate and dispense a lot of your beer.
6KG $217
12KG $267
For a disposable bottle
2.2KG $80
4.4KG $160
6.6KG $240
8.8KG $320
Even ignoring the fact that you could sell the bottle at a later date, using the above pricing, after using more than 4.4KG of gas, you would be better off with a refillable bottle.
I would go for a refillable bottle if you can afford it.