I just found the site of The Infrastructurist (subtitled: "America Under Construction") and a breakdown (with graphic) of transit infrastructure costs.
This particular article has an interesting break down of transit costs per mile for the following modes: intercity rail, street cars, light rail, heavyrail, and bus rapid transit. Wide ranges in costs are listed, from $1 million per mile to upgrade freight corridors in Ohio for intercity passenger service to $2.1 billion per mile to build the 2nd Avenue subway extension in New York City.
"Both Austin, Texas, and Hartford, Connecticut, claim they’ll be offering their citizens “bus rapid transit” in the next few years. But with Austin paying only $1 million a mile for its new service, and Hartford shelling out sixty times that much per mile, can the two really be put in the same category?
Actually, yes. This chart illustrates the enormous cost differences for transit projects. The variations, of course, relate to the characteristics of the proposed lines. For instance, in the category of intercity rail, Ohio is upgrading freight tracks between Cincinnati and Cleveland at a cost of $1 million a mile to allow for passenger service at 80 mph. California, by comparison, plans to spend $65 million a mile for a brand-new 220 mph high-speed system linking L.A. and San Francisco. Not surprisingly, constructing a reserved corridor for bullet trains is exponentially more expensive than making improvements to existing tracks."