The Trangie Caravan Park (NSW WESTERN DISTRICTS) does it by weight ... $2.50kg ... so an empty 9kg is $22.50, and an empty 4.5kg is $11.25. Thus any gas already in those bottles, you do not pay for twice.
But that's just one example. On my website there are hundreds of locations where you can get gas that cheap ... and cheaper ... as little as $12.00 for a 9kg refill.
The website is free service to the public, and I don't sell any merchandise or ask for email addresses. But I am scared to name the site in case the moderators ban me. As an 80-year-old grey nomad, I just want to help people save money.