Most groups charge fairly low basic rates and then have a whip around for unexpected costs.
That's how the group I've been in for over 23 years has worked most of the time. The guys who do least flying in my group are those who oppose raising the hourly rate - but they then put up their share of the emergency whip-round.
Only once (1993?) have we retrospectively raised the hour rate - ie you pay proportional to the hours flown in the last x months.
PS. Since we went on a permit, we've made decent profits towards an engine fund, with no whip-rounds needed. Currently £60 wet per tach hour and £50 per month for insurance and hangar, on an O-200 Jodel DR1050. Landings extra. Rate MUST be wet, on meter, to avoid problems.