The only source I have been able to find is 85 billion dollars but that's all religions. How much does Christianity, specifically, save because of religious tax exemption?
Much, much less than that, if anything.
The $85billion figure has already been debunked in this question.
Religious institutions would not pay $85b even if they were taxed as profit-making businesses. $85B would be around 3/4 of the entire amount of money donated to all religious organizations in the US. No entity pays 75% of its income in taxes - nor would it exist if it was taxed at that rate.