The most you can get out of SAT prep is +60-100 combined.
I'm not saying you shouldn't do it, but the Roslyn scores are real enough, well explained by the ethnicity of the test takers.
I'm not talking about Kaplan prep, which tops out at about $5,000. There is also whole mini industry of independent SAT coaches who charge significantly more and are said to get significantly better results. And, given the likely educational level of the parents, etc. that is a foot up, too.