I agree - I did a fair amount of SAT tutoring, and there's no question in my mind that preparation improves performance. However, all preparation is basically the same - take a practice test and see what you need to improve, then learn like a dozen tricks (on a math question you should maybe try plugging in the answers and calculating rather than doing algebra, get a good night's sleep, bring two pencils, ...).

These resources are essentially free - there's not much that an in-person class teaches that isn't available in a test-prep book from the local library.

Now, you need to know the book exists, go to the library, check out that book, read it, and do the exercises. Having a parent pay for classes dramatically lowers that bar (they'll get you out of bed and into a tutoring classroom on a sunny summer Saturday), but, unlike virtually everything else on a resume, money can't actually buy you that much of an advantage on the materials available.

It seems to me that a good standardized test score is one of the only things that someone with a lot of motivation and talent, but very few resources, can use to distinguish themselves.