Only invest the money if your kid will realistically invest the time. We didn't have the time - which can be hard to come by in a Junior year with AP coursework, activities and a job, so the money would have been wasted. (But she got into her first choice school and all the rest of the schools she applied for, so not really a big deal).
(Khan Academy has free SAT prep online if your kid is self motivated enough or can't do the SAT class schedule).