The digging is not as expensive as the foundation shoring, and is very region specific.

A lot depends on how often it is done in the area.

Its done quite a bit, I just don't know anybody personally who has done it in the past few years. Actually one client I worked with who had two adjoined houses did it in 2008 but doesn't know how much that cost specifically because it was part of a gut rehab. They also dug out one basement and filled the other in essentially, based upon existing conditions in each basement even though they were adjacent.