Look in the aircraft mags, find the price of these machines then divide by the number of shares (up to 20 as said), this will give the purchase price of the share. Running costs again depend on type and where kept.
We have a 5 member group around a PFA type kept at Goodwood, this works out at £85/month + fuel and landing fees (20l fuel/hour). We are looking to add a second CAA type (probably 4 seater), with 10 more in the group monthly costs will be about the same and hourly rates about £40-£50 so if you do a couple of hours a month you are saving
loads over club hire rates and with better availability- or, even better, you can afford to fly lots more.