Anything that relies on bag collection, from the bog that comes from the carp fisheries place, the late lamented BIAB, traveljohn, the popaloo, all charge a small fortune for bags and powders.
The best and cheapest I have found are/were* the disposa-john which buying 100 at a time worked out at around 35p per bag, contained the waste bag in a neat self-sealing foil bag and even came with a few leaves of toilet paper.
* I speak in past tense. I bought a shed load of bags back in 2016, was still using them in 2020 until along came Covid restrictions and out of my garage came my stored and once redundant Thetford 335 so I am now not up to date on prices, choices and sources of supply.
EDIT:
I have just done a search on dispoa-john and prices are quite ridiculous. I bought mine in wholesale quantities, boxes of 100 but now it seems to have become an easy to find retail item the price have gone through the roof.