Neither colleges nor high schools are ever alerted that cheating was suspected. Tom Ewing, an Educational Testing Service spokesman, said that confidentiality laws meant to protect minors prevented his company from disclosing that information. Of 2.25 million SATs taken every year, about 1,000 scores are withdrawn for misbehavior, 99 percent of which are for copying, he said.1,000 out of 2.25 million is a pretty low rate, but it sounds like the 1000 is people who misbehave during the exam and not people taking the exam for someone else, which is in some sense much more serious.
Optimal deterrent theory, of course, suggests that when the probability of detection is low, the punishment should be large. I suspect that ETS has a better idea of how common it is for students to pay someone else to take their exam. This sort of behavior has also been an issue with the GRE and the TOEFL.