Can you think of any economic and practical way for someone to get a 2-6KG CO2 cylinder, put it in the shed, and use it to refill their own small cylinder at home? That brewshop adapter looks pretty good, it's the inverse of one I bought - see below. That whole upside down, weighing thing sounds like a bunch of hassle, do they do that in paintball stores? I have no knowledge of filling tanks or anything like that, but I'm an engineer and I'm pretty sure I could be taught.
I've ordered one of these, which you attach to the top of a standard 9oz paintball CO2 cylinder so it can fit in a soda stream machine. Refills cost $10 at a hunting and fishing store in Lower Hutt, which is more than I expected, but I'm hoping to get some kind of loyalty scheme or something.
I don't mind experimenting a little and spending a little money to work this out. I drink 1.5L of sparkling water a day, given the savings over soda stream prices an investment would be repaid quite quickly. I'd rather not do a $500 experiment though. Soda stream refills aren't even that badly priced compared with buying sparkling water, I just don't like the lock in or them proudly saying on their website that the use the razor and blade model to make money.