Here's my $.02 on this topic:
My parents bought me a Kaplan CD-ROM that had both SAT and ACT prep 'courses' on it. They helped a lot - not because I didn't know what was on the test, but because they told me how the test worked, and they got me used to working with a time limit (lots of practice timed tests).
I spent, total, on standardized test review: $29.99
My score on SAT: 1370
My score on ACT: 35
It's not what you know or don't know (both tests just evaluate your elementary knowledge). It's how well you work under pressure and how familiar you are with the tests.