Discussion on:

Which countries deliver the most educational opportunity?

For starters, you have to divide the amount spent per child by the gross national product (or the average annual income, or something of that sort). This would take into account the cost of living in that country. It's just like in the USA. You can't take states where the cost of living is high, like New York, and directly compare how much they spend per pupil to states where the cost of living is low, like Wyoming. (And the same goes for counties, to some degree.)