We made the decision to send our son to a private school when he was entering 6th grade. He was in the gifted program in the public school system. We found a local private school to send him to for the next 2 years but then felt it would be better to get him transferred to a school for his high school years. We choose a school that was affiliated to a college and we were paying $6,000 a year and this was 18 years ago. This school had very small classes and the teachers and opportunities were great. I would have sent him there earlier if I would have know about the school. The school was a 20 mile each way trip for us and there was no transportation provided.
Another thing to consider when the child is older is that the kids were all enrolled in college classes as soon as they were considered to be ready. My son had almost a years worth of credits that transferred to college and he was able to cut back on the amount of years spent at college. There was 1 child there who is now a doctor that was really an eager student. He went to college his freshman year and had enough creidts accumulated from high school that he entered as a junior. So while private school can be expensive, college costs can be saved. We found colleges were very receptive to the private school graduate with scholorships etc. being offered to lots of them. So money spent now can be money saved later too.