Foxmoth &
Robin (good aircraft choices, BTW), I may not have made myself sufficiently clear - a common complaint.
My proposal was (and is) that at least every cost that the group would encounter anyway, regardless of hours flown, is built into the monthly fixed rate.
No estimates of usage are made when this figure is arrived at - any costs related to usage "lie where they fall" i.e. in the hourly rate.
That will inevitably result in the hourly rate being substantially lower, and will ensure that group members who don't fly much, but do have access to the aircraft with all the advantages that entails, still pay their fair whack.
Conversely, the velcro-gloved fiends who are never out of the thing
need also, as well as the fuel it takes to bore skyholes, to pay their element of the costs associated with requiring (e.g.) additional checks in any calendar period than would otherwise be necessary.
There will always be a balance to be struck, and it will likely depend (amongst other things) on the characteristics of the aircraft type (e.g. a Yak with a fairly low-lifed engine - which I presume is still the case - should ISTM logically have an element of the cost of refurbishment built into the hourly rate, rather than the monthly rate, so that he who gets the advantage pays for it).
Ultimately, I agree that the running costs should be kept as low as possible, but remain fair to all. If they aren't, friction amongst group members can easily arise and fester IME.