Jonathan Last has an interesting essay on the Taste Page of the Wall Street Journal this morning about the economics of the Duggar family, stars of 18 Kids and Counting. According to the essay, it may soon by 19 kids and counting.

Raising lots of children is privately expensive but publicly beneficial, Last claims, in that they help prop up our Social Security system. I made a similar point in my USA Today op-ed last spring, “Bring on the Baby Boom.”

But attempts to figure out exactly what it costs to raise children are always difficult. For instance, Last writes that “The National Association of Child Care Resource and Referral Agencies reports that in 2008 the average cost of a full-time nanny was $9,630.”

This is fascinating, because the federal minimum wage is currently $7.25/hour. It was slightly lower in 2008, but still. In New York, payroll taxes come out to about 12% of payroll. So to employ someone legally as a nanny would cost at least $8.12 an hour. For 35 hours, the minimum definition of full-time, this comes out to more than $14,000 a year. And most people who have full-time nannies do so because they work full-time, meaning they need someone for a few more hours than that. A full year of 40 hour weeks comes out to over $16,000.

Leave aside the issue of tax evasion (which is widespread, as I found out while hiring a nanny– legally– recently). What’s most surprising to hear is that the average nanny is working for less than the federal minimum wage! This is not part of the Duggars’ economics, but something is not computing here.