Every major infrastructure project that involves tunneling or digging runs into massive cost overruns, so basing the number on a cost estimate is already fishy. 100x is probably overkill, but not absurdly so. US infrastructure averages 8-12x more than elsewhere in the world, and it’s getting worse. New York adding less than two miles of track to their subway still cost more than double the estimate. California is spending infinite money on a rail line that may never exist.