There are lots of reasons to want to keep teens from getting pregnant: Girls who have babies earn less income later in life, they are less likely to finish high school, and they tend to pass along those outcomes to their children. Plus, given the gross racial and economic disparities among teen mothers, it's the kind of thing that tends to make America's inequality problem even worse.

It's also pretty expensive. For the taxpayer. Which means you.

According to a 2003 analysis, 79 percent of U.S. teen mothers receive some sort of public assistance: food stamps, TANF, WIC or housing. Then there are the higher incarceration rates of children born to teens, and the lost tax revenue associated with the reduced earnings that follow from educational underachievement tied to early parenthood. The National Campaign to Prevent Teen and Unplanned Pregnancy just released a total accounting of all the extra money the public spent on teen mothers in 2010, the most recent year for which figures are available: $9.4 billion (which doesn't include any of the costs that might've been incurred by older mothers, like childbirth). Even recognizing that more children generally leads to greater economic activity than fewer children, taxpayers might care about the extra dollars they're having to spend to take care of them.

The good news: That number is falling fast. The last time the National Campaign measured it, in 2008, it was $10.9 billion. The time before that, with 2004 numbers, it was actually less -- at $9.1 billion -- but that's because per-person costs of things like prison time and health care have gone up. So it's fair to assume that had the teen pregnancy rate itself not declined markedly over the same period, the cost to taxpayers would have been even higher.

The story is even better for some states. Costs dropped the steepest in Arizona, from $303 million to $240 million, or by 21 percent. The dollar numbers dropped most in places such as Texas, which has the third-highest teen pregnancy rate in the country (these estimates don't take into account the costs to states themselves, but rather the additional impact on federal programs like Medicaid).

Still, that topline number -- $9.4 billion -- is really high compared with the amount the public spends on family planning, which has been largely credited for the precipitous drop in teenage births. In fiscal 2010, that figure was only $2.37 billion, which is about as good a proof of the ounce-of-prevention aphorism as anything.