What's the future cost of getting a college grad to become productive? Nothing.
Future cost of making a baby productive? ~20 years of varying levels of investment.
Future cost of making a baby productive? ~20 years of varying levels of investment.
The common assumption that ajmurmann and vasiliys are likely working form is that value has been retained during the ~20 years of investment and the cost is proportional to the value and there cost is a reasonable proxy for value.
It is an assumption that does not always hold, but is common enough to do as a first order approximation.