Choxolate is spot on. It all depends on the value of the aircraft and the number of people in the group. A big factor that affects the value of the aircraft and the subsequent share price is how many hours does the engine have left? Also, what sort of state the groups bank account is in and whether there is sufficient in the 'engine' fund to cover an impending replacement or any big mechanical bill that is on the horizon? Bearing in mind that a 172 recon engine is probably circa £12k and a new one circa £20k.
Is the non-equity group that you have found £70 per hour 'wet'? Prices do vary region to region, but I doubt very much that you will be able to beat that price if it is £70 per hour wet?
To give you a comparison, I'm in a 172 'equity' group based in Norfolk and pay £65 per hour wet (plus £50 per month fixed and £135 club membership per annum).
Shares in aircraft are notoriously difficult to sell. So I would certainly consider a non-equity group if you can find a good one which also avoids the risk of any future unforeseen bills/expenditure.
Good luck!