From Denver International Airport in Colorado to Vail along I-70:(intermediate stops in Keystone and Frisco, and presumably Denver): $15 billion
From Fort Collins to Pueblo along I-25 (including intermediate stops in Denver and Colorado Springs): $5 billion
From here. The total would be about $21 billion (presumably a function of rounding errors).
One option under consideration would be to use electric trains capable of average speeds of 60 mph in the I-70 corridor and up to 140 mph in key portions of the I-25 corridor[.]
The $1.5 million study was done by a local government backed and state chartered authority.
In the mountains a key issue is the extent, if any, to which passenger rail could reduce the need for expensive highway expansion or increase state tax revenues associated with tourism. The only real comparison, and it is a poor one, is to the Ski Train (which was recently permanently cancelled and may or may not be uncancelled).