It sounds like your 182 is now playing the role of back up to the 182RG.
Also likely, it is a plane used by 172 users when they need more plane.

Also, it sounds like you have been treating the planes as individual units
up to now, and that no longer works for you.

Could you stop treating the planes as individual entities? Or could you
charge the other planes a "Backup Plane Support Tax"?

It seems this will be the cost of keeping the extra 182 around for
availibility reasons. Its going to need some funding from the other planes.
So instead of raising costs on just the 182, raise the costs on all the
planes a little bit.

This is going to be a big change for your club, but having a plane sitting
around under utilized costs money. It has to come from somewhere.