Kevkgd
The chap selling the share explained to me that comparisons with other aircraft don't work,
Well it certainly "don't work" in his interest! 
Some people buy an aircraft then sell 4 shares for a quarter of what they paid. So end up with one or two shares at zero cost. If you spot it you will get all the explanations you mentioned.
Is it worth the extra to you? Will it be worth it to whom ever you sell it to? Are you willing to take a lose on selling?
I know of a motor glider group that sound very like yours - its not "yours" but the same set up - shares can be had very cheaply as people often struggle to sell them.
I've been in really good groups, and one really 
one.