30W
Just to correct you there on the formula for CI.... CI = Cost of Time/Cost of Fuel so as you mentioned, fuel price increases, CI decreases.
Many airlines ignore variable cost index, due to either lack of resources or just can't be bothered with the hard work involved in using it on the line and so tell there crews to fly at a fixed index, no matter where they go.
In our company, we've actually fully introduced variable cost index flight planning alongisde optimal scenario analysis, and true enough, cost indexes are low and may add 5 minutes to your journey...the fuel savings at the end of the year are huge. Having an accurate cost index figure in the FMC/FMGC does many favours, it will determine what cruise speed to fly at based on that index, and modify it during the cruise depending on wind, weight, temperature etc. The descent speeds are around 255-270 kts at cross over which is sometimes too slow, and as you say, crews often modify this as they believe it's crazy to descend that slow, not knowing that this is where a lot of fuel is saved. Same in the cruise, pilots will often see M0.76 as the econ speed at a certain cost index, and ignore it, justifying there actions with garble about OTP when they'd be arriving in plenty of time anyway.
I think a lot of airline crews do need educating about the principle of cost index, especially when variable cost index is being used. It's good to know however that many of our competitors just peg around europe at a fixed index.