There have been a lot of posts over the years about airline cost indexes. But many of them have become extremely out of date. Please feel free to share the latest cost index information for your favourite carrier!

As you said it does depend on the airline, and the cost index depends on several parameters of the particular flight. I found an interesting, very informative article from skybrary.aero you may want to see.

 

I flew American's 737's and we had always used a CI of 60.  Fuel prices arose so then our default was somewhat around 40. We then started using a new system that would calculate the CI based on the data we entered, so the CI was different for every flight.

Can't give you a definitive answer though, sorry.