There's no definitive, publicly accessible list ranking the most expensive Christian schools in the US. Tuition costs vary significantly based on grade level (K-12 vs. college), the school's location, and its specific programs. Many private Christian schools don't publicly advertise their full costs, including fees and extras.
To find the most expensive, you would need to individually research the tuition and fees of numerous prestigious Christian private schools and colleges across the country, which is a considerable undertaking. Some of the most expensive private schools (religious and secular) tend to be located in affluent areas of the country.