There is research on this. I believe it was Thomas Espenshade, social scientist at Princeton, who had data on test prep access, modeled along with SES, etc. Tes

There is research on this. I believe it was Thomas Espenshade, social scientist at Princeton, who had data on test prep access, modeled along with SES, etc. Test prep is worth about 40 points on the SAT. Not trivial, but not a game changer. It's true that people who are otherwise well off are more likely to have test prep, and those people's test scores are often several standard deviations higher than low-income students from bad schools without test prep. But the counterfactual of (same person) +/- test prep = about 40 SAT points.