I've never actually been in a syndicate (merely looked at setting some up, years ago) but it seems to me that a lot of shares are being advertised at way more than the corresponding whole plane could possibly be worth.
Presumably there is an element of how keen the departing member is to get out. I have seen situations where a member wishing to depart had agreed (with another member(s) also wishing to depart) to not sell below a certain figure, and this will prop up the price - until somebody snaps and sells anyway.
IMHO a more important thing (than the cost of the share) is what kind of people make up the group. Most groups have issues of various sorts; a popular one is perpetual a VFR v. IFR dispute, where "IFR" avionics don't get fixed, and those groups see a steady stream of IFR-capable members who join and then jump off when they realise they will never get agreement to fix the stuff.