I think foxmoth you are trying to understand it too deeply. There isn't any sense to this business so there is little point in looking for some.
The price of a share is basically the appropriate fraction of what most likely punters might think is the market value of the plane, with a big variation.
Most of them are way overpriced in today's depressed spamcan market (because prices actually paid for used planes are way below advertised figures, but a novice buying a share won't know that) but they still sell because you are purchasing an entitlement to cheaper flying, compared to either self fly hire, or 100% ownership.
Anybody who can actually afford to own 100% should just sod it and do exactly that and save themselves all the hassle. When you are flat on your back with terminal cancer you aren't going to wish you spent LESS money on something you like doing.
And if you can't fly yourself often enough to keep the engine in good nick (say 1 hour per week) then you can always find somebody who can borrow it and give you some cash (make sure he's a "named driver") or even rent it properly.
I know of groups that work well but they tend to be close knit, made up of old mates and shares in those rarely if ever get offered.