It is normal for the value of the shares to add up to more than the value of the aircraft. This is supply and demand; more people will pay £8000 for a 6th than £42000 for a whole aircraft.
You are not buying into an aircraft; you are buying into an active flying group. This will include “value” paid for hangerage, insurance, engine fund, general bank balance, spare parts etc. This can add up to a very considerable amount. Some groups which are very stable so rarely sell shares may undervalue this element.
On aircraft value, it is always useful to find out what it is insured for and why.
If you have the cash and can handle the hassle, it is possible to buy your own aircraft, write the rules, sell shares and pick who you want in the group. I have done this several times with considerable success.
Rod1