Dropping out of college after a year can mean lost time, burdensome debt and an uncertain future for students.

Now there’s an estimate of what it costs taxpayers. And it runs in the billions.

States appropriated almost $6.2 billion for four-year colleges and universities between 2003 and 2008 to help pay for the education of students who did not return for year two, a report released today says.

In addition, the federal government spent $1.5 billion and states spent $1.4 billion on grants for students who didn’t start their sophomore years, according to “Finishing the First Lap: The Cost of First-Year Student Attrition in America’s Four-Year Colleges and Universities.”

The dollar figures, based on government data and gathered by the nonprofit American Institutes for Research, are meant to put an economic exclamation point on the argument that college completion rates need improvement.

But the findings also could give ammunition to critics who say too many students are attending four-year schools — and that pushing them to finish wastes even more taxpayer money.