Actually, this past year, the SAT changed, and the average SAT score dropped for the first time in 31 years. Test writers are assuming it's because the average test taker takes the test more than one time...and this was the first time the new tests were offered.
My daughter is a new junior in high school. She will take an SAT prep course because her GPA is high. That high GPA combined with a good SAT score will be the difference between geting free tuition and getting a completely free ride in college.
So what difference does it make to take it more than once??? I'd say...at least for me...about $50,000.
By the way. If you have a Junior in high school, you need to pay particular attention to preparation for the PSAT. National Merit finalists, semi-finalists, and commended students come from those scores....and that is a nearly guaranteed free ride in college.