"Reducing the operating costs for the co-owners".
1. Could mean that more than one person owns the aircraft.
2. One or more of the owners is looking for someone else to share the running costs that are not being covered by the hire rates/charges for lessons.
The correct value will depend on:
1) Airframe Hours and Age
2) Condition
3) Equipment fit
4) Engine Hours remaining
5) The state of the market
This is too many variables to get a reliable answer without posting further information.
"After two lessons" - Interesting! If the Advertisement consists of verbal statements along the lines of "It's a steal; too good to miss; you're a natural; you'll need a four seater once you qualify" I'd run a mile. If it has just appeared as notice on a notice board, and been the subject of idle chit-chat, they are probably just trying to sell a share. Treat as a commerical venture if the aircraft is a 'working' aircraft. Either way have a licensed engineer examine the aircraft before parting with any money, and use an engineer with no relationship to the club.