I never read the whole story, only having heard parts of it. I know there were some problems building an interstate tunnel underneath Boston but why would it be 12 billions dollars worth? Unless 90% of the federal money is lost to corruption.
They built two tunnels under Boston, another tunnel under a shallow part of Boston harbor crossing a subway line, and a new cable-stayed bridge. And they did it all without disrupting the existing highway. If you can tell me you can judge the correct cost of that without pulling a number out of your ass, you're a liar. ;)
A lot of money did go to install new windows in everyone's home in the North End to compensate for several years of jackhammering and trucks rolling by. But corruption isn't it. Incompetence and a steep learning curve cost far more--no one had ever done anything like this before. So what if Boston will get credit for no one ever trying anything like this again. :)
As another note, that $14.6 billion is the total cost to everyone, much of it borne by Massachusetts drivers and taxpayers, and the federal cost is only part of it. Our state still pays in way more than we get back from Washington every year--something like 21 cents on the dollar goes to states like Oklahoma, New Mexico, Mississippi, or Virginia instead of coming back to us. Even at the height of Tip O'Neill's reign, we barely broke even on our federal contributions. The Big Dig was expensive but traffic flows through the city like a babbling brook, people get to the airport much more easily, and the ugly old el that cut a scar through the city is gone forever. Definitely an improvement.