Bernie Sanders argued that public college should be free during the Democratic debate on Sunday, saying that he would finance free tuition by "imposing a tax of a fraction of a percent on Wall Street speculators," according to his campaign website.

How much would such a plan cost?

Here's a quick estimate: The College Board estimates the cost of a four-year, in-state public college, including room and board, to be $19,548 for the 2015-2016 school year. There were 4.85 million students enrolled in such programs in 2013, the last year for which the College Board calculated this number. That's about $95 billion. Without room and board, it's about $45.6 billion.

This is an oversimplified estimate. Current tuition doesn’t necessarily reflect the actual cost of education, and the number of people who go to college would likely increase if it were paid through taxes. This estimate also doesn't reflect the cost of two-year programs (Sanders has not been specific about whether he means four-year colleges or two-year colleges or both, but his opponent, Hillary Clinton, argued that community colleges should be free, while four-year universities should be "possible to attend" with "debt-free tuition").

Sanders's campaign estimates that the plan for free college would cost $75 billion per year.