Well, even better, you don’t even need to purchase them—most public libraries have SAT prep books available for borrowing.

When I was in high school, I would check out whichever ones were available at the time and would photocopy the whole book. I drilled myself on the tests until I scored 1600s every time. Shows how flawed they are as a testing vehicle.

That's not evidence the tests are flawed. I was just having a very similar conversation with a friend who aced the physics olympiad. She said, "it's too easy to game, all I had to do was try old tests on my own". Well, hundreds to thousands of others were putting in more hours doing the same thing, in addition to enrolling in expensive prep programs, and they didn't do as well as her.

That is evidence in favor of the test's quality, not against it. There's no paying to win.

That’s a good point. You’re right if the aim of the test is to measure information retention, and not raw intelligence.

Never mind what it measures. My main point is that whatever it is, it's not simply "dollars spent on prep". Good tests provide an opening for the less well-off to succeed, despite the more well-off doing everything they can to rig it in their favor. My parents (and by extension me) owe a lot to them.