Generally group shares are priced above the percentage valuation of the aircraft. There would be good reason for that if there was a good engine fund or other assets. But in most cases it is simply supply and demand. The smaller the share cost the easier to over price it and still sell the share.
It gets even worst if you try and buy a group aircraft - then there will always be one or two members who are sure the aircraft is worth more and are almost bound to nix a sale!
So as a general guide - the percentage of the aircraft value (which is frequently much less than a group thinks) plus whatever premium a group think they could/should charge.