“Congress shall make no law respecting an establishment of religion or prohibiting the free exercise thereof.”

Taxing churches would potentially violate the clause "...or prohibitimg the free exercise thereof" part of the amendment and would most likely not survive a constitutional challenge (especially in the SCOTUS we have now).

The founding fathers put that language in there for a reason: The power to tax is the power to destroy and they did not want the government to have that power. They were wise men.

By the same logic, the $71,000,000,000 is not a cost to the government. You can only make that statement if the government had a legitimate claim to that revenue. Absent the power to tax it, there is no claim to the money and no basis for calling it a cost to the government.