In a group the 'income' (i.e. cost to members) is used to pay the 'expenses' (i.e. money flowing out of the group to keep the aircraft flying). The income is generally split between an hourly rate and a monthly charge where the hourly rate is notionally based on the direct cost of operating the aircraft for an hour (including share of 50 hour checks etc) and the monthly charge covers all calendar based costs such as insurance, hangarage, the annual etc.
There are two ways to look at the cost of something comlpex - try to find out all the individual costs and add them up OR look at someone else who is doing the same thing and copy them. You started the ball rolling with the 2nd method and I am running with it.
To come back to the example, the assumption is that if a group charges themselves the equivalent of £8400 a year on top of the hourly rate, then this is what the group expects to pay out, averaged over the long term.
There are many ways to tweak this model to suit the needs of the group, particularly regarding how the cost of a new engine will be funded, when required.
A couple of ownership costs that are often ignored are depreciation and cost of capital. Cost of capital is either the cost of borrowing the money to buy the aircraft or (if you already have the cash) the loss of income from anything else that you might have done with the capital (opportunity cost).
Depreciation is beginning to hit traditional light aircraft as the cost of composite equivalents become popular. It is therefore critical not to pay over the odds up front.