Rough estimate is 45 seconds per hour for each .01 Mach or 1.5 minutes per .02 Mach.
As others have mentioned operational decisions (on time, late, connecting passengers, etc) can result in different cruise speeds.
Headwind vs. tailwind also has an impact. Higher than expected headwinds might result in higher CI/cruise Mach to avoid late arrivals and resulting operational difficulties.
Boeing article on Cost Index (CI). First 10 +/- pages cover general concept and cruise applications -
http://www.boeing.com/commercial/aer...ConsSeries.pdf