How much money are you saving by refilling your own?
Dunno, its just part of the 'fun', but a quick look at costs
Lets say a 100g cartridge is around £3 - £4 (lets call it £3) so 1 kg of gas costs £30.
A 15kg Butane cylinder costs around £25.
15 kg would give 150 cartridge refills = equivalent cost £450.
Alternatively, to fill from 500g cartridges.
Amazon has 6x 500g Coleman cartridges at £24 (inc Delivery)
So I can potentially fill 25x 100g cartridges which would have cost £75
The non-financial benefit is that you can carry a cartridge of the correct size (knowing it's full) for the 'trip' without having to carry an 'extra one' because they may not be enough in the part used one, or ending up with loads of part used cartridges sat in the cupboard.
I like 'pottering' about, making stove-stands, paracord water bottle carriers, looking for ways to reduce weight etc etc.