Where we live, in a Tiger mom rich area, lots of highly educated parents, it is very common for parents to hire private tutors for their kids in high school and junior high. My friend hired a math tutor for his 5th grader, who is "good at math," but they want him to be better.
When it comes to the SAT, these kids will outperform the poorer kids who may be just as smart, since they will have access to better study prep resources.
My son got accepted to our local community college's summer gifted program. He will take 5 weeks of classes there in math, science and literature. The classes are pricey ($300 a week for a half-day class, ouch), but we think it is worth it. His best friend also got accepted but the dad is a truck driver and they just don't have the money for it, so he will stay at home this summer and read, etc.
Having financial resources can obviously help your gifted child, if used wisely. That isn't to say that poorer gifted children also can't do well.