At least one poster suggested that the accounting manager showed the amount of CO used far exceeded the size of the cartridge... in other words, it would appear that the "wastage" was just a problem with the accounting software. I don't know if anyone else has verified that.
In my case, printing almost entirely B&W on gloss paper, the CO is being used at about the same rate as PBK, GY and LGY, which seems credible. I haven't yet reached the end of any cartridge however.