I don't think value for money is possible, partially because you can't compare. No one ever knows how any given child would have done in another environment.
We have ended up paying when we didn't start off intending too, but unless we move areas, will probably continue to do so. This is because we think the average or slightly above average child ( which, frankly is A LOT of the population) will almsot certainly do better with smaller classes, more breath, more options for depth and high expectations/aspirations.
The super bright all-rounder will do well anywhere.
The internally motivated child will achieve their potential anywhere.
I'm not sure your more average, middling to low motivated child will.
Through no fault of the teachers, the state school ds1 started in (we moved him as he hated it, rather than for academic reasons at that point) seemed entirely focussed on getting everyone to a. minimum standard. This takes up so much time - in his prep he is getting his basic maths, literacy etc but also dedicated specialist teaching in music, ICT, history, geography, french. With the exception of french, ds1 absolutely loves he detail, range of topics and having lots of teachers who know about their subject. He is struggling with writing, and getting 1:1 help with this and self esteem work twice weekly has been very beneficial.
We are making sacrifices (house size primarily) to provide this but have decided that it is hte best option for ds at present.