Hi there.... email me offline at
[email protected] and I'll send you a spreadsheet I've done for a C172. You can adapt it to a C150 no problem.
You'll have two lots of costs that you'll want to play with....
Fixed
Hourly-dependent
The BIG cost is engine replacement, which you need to budget for.
Typically, a coupla C150/C152 groups up at Barton offer (I think) £30-£40 pm + £25-35 per hr wet. Those are based on 20 share groups (i.e. 5%).