A presentation to MBTA (The Massachusetts Bay Transportation Authority) board estimates that, connecting the Red and Blue Lines of Boston via a 2000-foot tunnel (so-called `Red-BlueConnector) will cost $850 million and take a decade to complete.

Presentation slides at MBTA website:

https://cdn.mbta.com/sites/default/files/2021-06/2021-06-07-fmcb-O-red-blue-connector-update.pdf