How much does it cost to deliver a unit of education -- say, a semester of Algebra I -- through a computer? How does that compare to the cost of doing the same by means of a teacher in a classroom? Is it essentially the same product? And if it is cheaper to explain to teens how to solve for X using the tools the computer age has given us, who should realize the savings?

These are pressing questions in Pennsylvania, where the state association of school boards has asked the state to put a moratorium on the formation of any new "cyber charter schools" while districts try to get a handle on the issue. Right now, Pennsylvania schools pay about 80 percent of their per-pupil costs to cyber schools as tuition for students that are registered in their district but enrolled in the online schools. But the bricks-and-mortar crowd isn't convinced that the online schools face anything like that amount in costs, given the lack of large school buildings to staff, maintain, and heat.

Pennsylvania districts paid about $368 million last year to the state's 144 charter schools, which enroll about 50,000 students, according to state records. But the state doesn't break down those payments between regular charter schools and the on-line variety.

With public education dollars always in high demand, the questions being asked in Pennsylvania are probably going to come up over and over again in other states as the on-line education industry matures. The key to getting answers will be requiring cyber schools to provide detailed, accurate accounts of their operating costs and procedures, and having those accounts analyzed by people with expertise in both education and learning technologies.

The payoff for students could be huge, as we learn the best way to combine on-line and traditional educational models to meet the needs of different kinds of learners.