liquidhockey and yawningdog a 1/10 share in a C150 might cost around 1,500 - 2,000. Genghis was referring to the total cost of the aeroplane I think (they'd be very expensive shares otherwise!!!!). A decent 172 or early model PA-28 (pre-Warrior) can be had for around 25-30,000 dependant on type, age, engine hours etc as Genghis said.
As suggested, check out the usual places; Pilot, Flyer and Today's Pilot magazines. They all have a section for shares.