dublinpilot is exactly right in his financial analysis; however I doubt most share buyers look at it that way.
Most of them buy a share because it's a much cheaper way to get to fly than self fly hire, and they can't afford to buy a whole plane.
If everybody had say £30k+ to spend on a plane, there would be virtually no market for shares in spamcans. Whereas loads of people have say £5k - in general if you can't dig out 5k somehow then you are either unemployed or you have ended up in a tight situation where every penny is going straight out elsewhere.
This is where the market for shares (at the price level we are talking about) comes from. The asking price doesn't get analysed for value, and IMHO if it did get analysed for value then share prices would plummet. Most planes are worth a lot less than the share price x # of shares, and most groups don't have a fund anywhere near big enough to go forward without major top-ups.
And there isn't much choice of shares to buy. You can buy one only if there is one for sale, and they usually come on the market because somebody is unhappy with the group.