You don't need a PT CofA if you own the plane or part own it
I think you will find this is
wrong!
You are OK if you own the aircraft yourself (and also OK for owners immediate family), but if it is a share you
do need the PT CofA unless the instruction is done as a private flight and the instructor is not paid (ref -http://www.caa.co.uk/docs/33/CAP393.PDF)
f money is an issue so much is buying into a potential bottomless pit right for you?
As a part owner this is not usually a major problem as (especially in a larger group) the bills are shared and the reduced flying costs likely to more than compensate if you do get this happening (after all, the hirer gets these bills and just builds them into the hire rate)