But I doubt seriously that freeway-class roads, even in D.C., would be as catastrophically expensive as Metrorail has been so far.
Do the math.
I did.
Maryland's Md. 200 (ICC), built at about the same time in the same labor market as Dulles Rail, came in at about $2.4 billion (the final bill was a little less than that) for about 17.65 miles of new freeway-class toll road and supporting infrastructure (including interchanges, widening of I-95 and two maintenance centers). That works out to about $136.1 million per centerline mile.
Dulles Rail is about $5.7 billion (as of right now) for about 23 miles of new rail line, or about $247.1 million per mile. Much of the right-of-way for Dulles Rail was "free."
For starters, any such freeway would require a far wider right-of-way than a Metrorail line. Or would require wider tunnels. Then there's the ROW costs for that wider ROW. Nevermind the ancillary impacts, like the need for more parking downtown. Where are you going to get the space for that, and at what cost?
Tunnels should not require much on the surface, as long as they are not cut-and-cover (I-395 under Third Street, N.W. was cut-and-cover, as was much of the Metrorail system in downtown D.C.).
This would not be a parking project. And since the District of Columbia collects a large amount of parking space tax on private parking, and generally does not fund parting space construction, it's really not relevant to this discussion.