My approach is that the whole idea is to exchange postcards with everyone else. I do not see anything under 50 as being difficult to manage; at least, not for people using RC silver paper - anyone using fibre or Alt Proc would obviously spend a much longer time in the darkroom.
Cost to me is about 3 hours in the darkroom (I don't see that as an negative BTW!) and about 30 quid once every 3 months on paper, chemicals and postage. I consider both to be time and money well spent.
Some 46 people were happy to proceed on that basis in the last round. See what happens in the next round - I suspect we will get a similar number as some drop out and others drop in.