The New Jersey Office of Legislative Services, which estimates the budgetary impact of proposed laws, has just released its analysis of a private school choice bill called the “Opportunity Scholarship Act.” The most remarkable thing about its report is the amount of money it assumes that districts would save for each student they no longer have to teach: $0.

On that assumption, if every student were to leave for the private sector tomorrow, districts would keep right on spending exactly the same amount they spend today. Inefficient though it is, not even state-run monopoly schooling is that bad.

The OLS report does not explain why it assumes that the per pupil savings for students leaving public schools (the “marginal cost”) would be $0. It states that this figure is “indeterminate,” but by not counting it at all is effectively treating it as zero.

In fact, the marginal cost of public schooling is not “indeterminate” at all. Economists “determine” it all the time, and it’s quite easy to do. You simply observe how district spending actually rises and falls with enrollment, using a time-series regression, as I did in 2009 to calculate the marginal cost of public schooling in Nevada (see Appendix A).

Even if the NJ OLS does not conduct a marginal cost estimate specific to New Jersey, they could have done–and should still do–the next best thing: take the marginal cost estimates for other states as a rough guide and estimate the NJ district savings from them. I estimated that Nevada district spending falls by 85% of average per-pupil spending when a student leaves, and Grecu and Lindsay, a couple of years earlier, estimated the figure at 80% for South Carolina.

If they want to be conservative, the NJ OLS could use the lower of these figures, and perhaps also run the numbers for estimates 10% higher and 10% lower.

Any of the above options is preferable to the logical impossibility of their current analysis, which effectively treats the marginal cost of public schooling as $0.