Anecdotally, I write this comment from the couch of a poor family. My girlfriend’s parents (who I am visiting) live on welfare in a 1 bedroom apartment (hence the couch we are sleeping on). They have a 15-year-old son who could potentially benefit from test prep. He’d also benefit more immediately from a haircut, new clothes, and a therapist. If the parents had $1000, they’d probably buy all of that instead, or lottery tickets.
Extra anecdotally, I also have a 15-year-old stepsister, who has a rich mom. She has new clothes, a nice haircut, and antidepressants. Soon she’ll have a car for when she’s 16 and most likely will revive some SAT prep. I have noticed (or at least have theorized) that because my girlfriend’s family cannot afford these things for their children, her young brother in a state of arrested development. His lack of luxury is causing him to miss certain life milestones. His maturity is stunted (even for a 15-year-old boy) and he would not be a good candidate for test prep.