The Commonwealth of Massachusetts is supposed to build a connection between the Red and Blue lines. But they really, really don't want to. It's probably a very good idea, since it will increase mobility for Blue Line riders accessing jobs in Cambridge. And it will take some of the capacity crunch off the core subway lines where Red-Blue transfers take place. And it will improve access to Logan Airport. And it will shorten trips for thousands of commuters each day who will no longer have to transfer twice, in already-crowded downtown stations. There are definitely benefits. And the state is supposed to build it.
They're doing their best not to. They've claimed that the construction would cost $750 million, which has been called "deliberately high." How deliberate? The costliest subway project in the country—the Second Avenue Subway in New York—costs 1.7 billion dollars per mile. The Red-Blue connector is 1500 feet long. Less than a third of a mile If you do the math, it comes in at $2.7 billion. That's not even in the ballpark. It's an obscene figure and is risible. It shows the state does not want to come close to making a good-faith effort to support this transportation connection.
How much would it cost? Well, likely far less. The current Blue Line tunnels extend underneath Cambridge Street to a former portal at Joy Street. Continuing the tunnel would certainly be disruptive, but could be done in a cut-and-cover method without fully disrupting traffic flow (the street is five lanes wide including the median, and the subway would only require two). Utilities would require movement, but there would be no need to support an elevated roadway or surface train lines above (like the Big Dig). Traffic on Cambridge Street would suffer for a time, but better transit service in the end would be worthwhile. Alon Levy has a compendium of subway construction costs, and most projects fall under $1.5 billion per mile.
A good analog might be the Central Subway in San Francisco. It's a short extension of a light rail-sized transit line (as is the Blue Line; the East Boston tunnel was originally built for streetcars so it's a much smaller gauge than, say, the Red Line) extending to a terminal station in a dense city. It is budgeted for $800 million per mile. If the Red-Blue Connector was built for the same cost, it would come in around $250 million. Or a third of the cost the state estimates.
The $750 million figure is not disingenuous. It's a lie. And an elevated connection—with only a few hundred feet of elevated structure—could be built for a fraction of the cost with a better final product, to boot.