The discussion of how much money to spend on education should not be done based on some expected value of education. It should instead be done by polling various interested parties on how they feel about current the current education system, what resources they are missing (if any), what types of spending are not producing useful results etc.

You don't have to quantify the expected value in order to decide on costs - this is only done as a reductionist homo economicus type model.

For a more direct example, when deciding to go watch a movie, do you do some computation of how much your net worth will increase by virtue of seeing the movie, and compare that to the ticket price? Or are you simply acting on practically unquantifiable subjective feelings of enjoyment about the movie, and comparing the price to entirely different goods (e.g. you probably wouldn't go if the ticket price is 10000$, because you can buy other things you feel are much more enjoyable/important with that kind of money).