Just reading this thread with interest.
I am currently contemplating a share in an 8 person syndicate.
I have seen two of the same aircraft type (Motor Glider) for sale at £12k on AFORS & Glider Pilot.net. So as a bench mark this is a start.
However, the share I am being offered (in the same type of aircraft) is for £2.5k. As this is for a 1/8th share that values the aircraft plus syndicate assets etc at £20k.
The chap selling the share explained to me that comparisons with other aircraft don't work, as buying an aircraft with no where to put it (locally to where I live) might be a problem. PLUS it's an existing and very well-run syndicate with spare engine and prop, parts etc pluis some cash in the bank, there are also skilled members who are able to do the work on the aircraft themselves etc - a different proposition altogether. There is also a premium for being able to keep an aeroplane at a site with excellent facilities and runways and cheap hangarage (no more space available at this local site to me either)
So, this is the advice I've been given, just thought I'd pass it on.
For me it is ideal and convenient to where I live.