Fleet flyer
Your maths is typical of the aircraft market at the moment in that you are fooled into thinking that a C152 is worth lets say £19K.
It that is maybe all you want to pay and for that all you will get is a very worn out old dog (unless you are lucky enough to find a forced sale)
It terms of the reliability you need for a commecial operation you will need to invest in the overhaul & maintenance of aircraft so it is quite likely that to the owner of this club the aircraft is worth £30 -35K in terms of outlay and expected return and is insured as such.
So the reason that the market thinks that you should pay £12-19K for a C152 is because all the good aircraft are not for sale.