Myself and a few others on here have been doing this for ages, not to save money but to have the reassurance of always having a near full cylinder on board. The tare (empty) weight of the cylinder is about 3.7kg and is stamped on the base: you just have to fill it to that plus 2.7 kg* for butane. You can fill with propane and some Campingaz cylinders are stamped to indicate they take 2.5kg* propane but you seemingly can't top up a part full propane cylinder with butane as the pressure is higher. All parts are available from BES plumbing supplies - pm me if you want details.
* I always limit to 2.5kg butane / 2kg propane.View attachment 94893View attachment 94894