More text on the return to service...
BBC News - Boeing 787 Dreamliner returns to service in Ethiopia flight
If, as reported, Boeing invested 200,000 engineer-hours on the fix, and estimating a burdened cost per engineer hour of, say $200, Boeing's labor cost alone for the fix is around $40M. Ouch.
And that figure is probably much less than the lost revenue to the carriers who operate the 50 grounded Dreamliners, not to mention all the airplanes whose entry into service was delayed. Double ouch.