We went through similar decision-making process two years ago with my now first grader.
We opted to have him tested to help prune the decision tree. The tester recommended three gifted schools in the area -- but like you, we found the $30K+ price tag prohibitive. At the end we settled for a small private Montessori school. While there were some rough patches in the Kindergarten year, first grade has been a very positive experience so far. Classroom is mixed-age so socialization is not an issue. Subject acceleration is built into the system and my child is adequately challenged.
In our case a whole grade acceleration in a single-age classroom would not have worked because of maturity issues. I would not recommend ChallXXXger/StrXXford for gifted children. PM me if you want specific names.
