The cost index setting is not the point.  Regardless of what the cost index is set at, the commanded FMC speed is not being held by the A/T system.  I set a c/i of 30.  The fmc said the cruis speed would be .78 and the A/T system kept the plane at a .82 +/- .02 mach.  I can set the cost index to 1 and the A/T sytem will not keep whatever the calculated cruise speed is.

In other words, no matter where the speed is managed from, the MCP, the C/I figure, the VNAV cruise speed entry...no matter how the speed is set, the A/T system is not keeping that speed with any degree of certainty or accuracy.

As to what airlines fly what cost indices, 80 is very normal according to this site, though I have zero info on the reliabilty of their sources.  80 seems to come up on most sites for American Airlines...
https://costindex-index.fandom.com/wiki/American_Airlines_Cost_Index
https://www.scribd.com/document/374519450/Cost-Index-Database-2017