I am sure there is a formula of litres to kgs or per 100 grams. The cylinder could be weighed before and after. Bunnings price of $18 swap for a 8.5kg is reasonable compared to others that charge in the high $20 or even $30.
An 8.5 kg bottle will hold about 17 litres of LPG or about $14 @ .80c per litre.
sure - so which business is going to operate calibrated scales and risk the wrath of weights and measures etc when a bloke complains he was ripped off 100 grams?
don't get me wrong, no one like to pay for something they haven't got 100% of - me included.
the packaged swap products are weighed on calibrated scales when filled, and guarantee the weight according to the advertised weight on the packaging.
when you get yours filled, invariably you get more in the cylinder than the packaged product would - I think I posted about this once before - its all to do with safe filling capacity and expansion.
as filling is cheaper than buying/swapping packaged, you really get more for your dollar - if you follow? assuming you have only a small amount in the bottom.
actually, we are all very lucky that refills are still allowed - overfilling is not dangerous per se, but could be in the right circumstances.