What about the demands on the time of professors from students who repeatedly try and fail the exams? Seems like that could get expensive for taxpayers.
It did happen quite often to have to retake a few exams. Some students could never clear some particularly hard ones. But I have never heard any professor ever complain about it. Nobody wants a struggling student to finally succeed more than the professor who has to keep failing them.
I can't imagine marking the exams was a significant part of the professors time. Especially because its probably the sort of thing they could get an assistant to do.
Indeed, evaluating an exam does not take that much time that it would be considered expensive.
That said, in my country (in EU), it's not indefinite tries. The fourth and later attempts (up to 8, I think) have oral exams in front of a panel of professors so that they can evaluate if there is actual lack of knowledge or if there is some sort of problem between the student and the professor. The student must by then pay for the attempt to pass the exam. It's not expensive, though, it's just another incentive to make the student really be sure to be prepared.
I've known this to differ between professors as well as exams. Lower levels and higher volume ones were almost always marked by assistants. The oral exam was always done by professors, though.