In Standards and Spending - Dispelling the Education Orthodoxy, Dr John Marks explains that he got this figure by comparing the results of the top 100 independents with state sixth forms, crunching up fees and funding, and spitting out the result in what the independent-school spokesmen huffily call "a crudely mechanistic way".
It is part of Dr Marks's argument against the namby-pamby liberal line that spending more on education will improve it.
The story was flagged in one paper by the unsurprising headline "State schools better value for money than private". Well, we knew that. Quite apart from obvious economies of scale, it is axiomatic that if you put any mixed gang of top headteachers in a room together, sooner or later the private-school ones (shadowed by beady-eyed bursars with notebooks) will sidle up to their state-school oppos and hiss: "How the hell do you do it on the money?" The comparative meanness of spending- per-pupil in the state sector is legendary. Cash-strapped private schools gnash their teeth at the mystery of how well the good "bog-standard" comprehensives manage.
But really, it seems a cruel irony that the penny-pinching efficiency of state schools should be used against them in this way, to conclude that there is no point whatsoever in bringing them closer to a private-school spend.
The only attractive feature of the CPS report is the idea of introducing value-for-money in league tables. I back that simply because anything which makes the accursed things more obscure is to be welcomed. Then parents can go back to choosing schools on the basis of gut feeling, local reputation, and whether or not the headteacher gives them the heebie-jeebies.
The trouble with the calculation over A-levels is twofold. First, you can't price an A grade because some of the pesky little varmints just won't get one, however much you spend on them. Pay pound;30,000 per subject and still some will be too idle, too slow, too bored, and some unreasonably spooked by Edexcel accidentally setting the question back-to-front and losing page 3 down the back of the sofa.
The other problem is that the high spend of private schools does not all go on a kamikaze rush for high grades. Oh, they want them all right: they lust for them, they coach and coax and police the coursework deadlines.
But that isn't all they spend it on. There are other things which are unfairly lavished on private-school children while being denied to their state-funded counterparts. Things like music lessons, with plenty of instruments available, well-stocked libraries and laboratories, quiet chapels, playing fields, swimming pools, grass, fresh air, fabulous IT, and pleasant classrooms in decent architecture with most of the paint still adhering to the walls .
There are small classes led by teachers who stay a long time, are not driven demented by paperwork, and have a reasonable chance of knowing everybody's name. There are abundant school trips. There is individual help.
There is also - in the less fiercely selective schools - often a better chance of chivvying the duller and more obstreperous pupils into at least some success.
Never mind the A grades, what about the triumphant and unexpected Cs? That's the real measure of a crack school. We all know privately-educated kids who, without huge amounts of expensive attention, would have quite understandably been despaired of or thrown out, but who are miraculously prodded through two A-levels.
Money is spent on all these things in private schools, because if it wasn't the parents would cut up rough. They're clients. In the state system we are also clients - all of us, with or without children - but it is harder to crack the economic whip.
The high pupil-spend of private schools might not, in the end, buy your darling a row of straight As, but it can help to provide other things: order, space, peace, personal attention, music, sport, variety.
Yes, it's unfair that some people can buy their children these things; but it's unfair that some people live in luxury and hire personal trainers, or have wider-screened TVs than the neighbours. And yes, there are lousy private schools (close them down) and yes, the best state schools also manage the peace, music, variety etc, often in catchment areas which make the achievement miraculous.
But there is something really creepy about suggesting that we should not bother to put better resources into looking after state pupils , just because it might not guarantee more A grades. Ugh.