I think it is easy to start being picky about costs, but at the end of the day there are many variable to the mix. Print size, type of paper (yes, some papers require and actually use more ink than others), etc etc etc. So I just think of the costs as being in 3 categories: An Epson 2200/2400 is 'x', a 110ml cart is less than 'x', and a 220ml cart is less than 110ml carts. So it is all relative.
Although I appreciate MardDS's contribution to ink cost per print, it is way out of the scope of what is useful for me. Photography is an exercise in creativity and output, as opposed to an engineering exercise. At the end of the day I just think of each of my prints as costing me a few bucks, give or take a few bucks.
Mark, this isn't a slam at all on your calculations. I just has to do with my particular take on the whole exercise and how it may (or may not) related to my particular interests. That's all.