A union representing American university faculty claims that online instruction is more expensive than traditional face-to-face teaching.
A report from the National Education Association argues that class preparation takes longer for online courses than for traditional programmes and that it is cheaper and less time-consuming for faculty to interact with students in person than one at a time by email.
"Online courses should not be thought of as automatic money-savers," said Bob Chase, president of the association, which represents university faculty. "In-person instruction will always be less expensive than instruction via the internet."
Mr Chase said online education worked only in small classes, with lots of preparation time, which did not come cheap.
The union's report contends that "administrations often overestimate the impact of technology on education and assume that it will both automatically lead to more effective teaching and save the institution instructional costs".
In fact, the report says, an hour of instruction on the web takes 18 hours to prepare, versus two hours to prepare a one-hour lecture for the first time. It also said it is more cost-effective to provide in-person lectures than distance-learning courses.
However, Lanny Arvan, director of the Center for Educational Technologies at the University of Illinois, is sceptical of the union's figures. "I will concede the point that online instruction involves more faculty time," he said. But he added that on overcrowded campuses teaching over the web was more economical than building classrooms and hiring more faculty.
The University of Central Florida in Orlando, for example, is growing so fast that it is holding classes in cinemas. By putting many of its courses online, the university has avoided the more costly option of expanding the campus.
At Drexel University in Philadelphia, a comparison of an identical graduate programme offered both online and in the traditional way found that the cost was virtually the same.
Dr Arvan said that accommodating distance learning had less to do with money than with shifting workload to accommodate the greater preparation time.
"There's a faculty time allocation issue here," he said. "If other obligations are not reduced and there's not a huge amount of support, then the union's right. But I would argue that that's more of an incentive issue than a cost issue."