CJD,
On my Epson 4800, over the period November 2005 to yesterday, for the same 1600 sq. ft. I mentioned, the grand average without cleanings works out to 0.013 ml per sq. in. Of course, any one in business needs to add-in the overhead for both cleanings and waste prints. I've written articles and provided the model for doing this on this website. Once I include the cleaning experience I mentioned above, the combined ink usage is 1.01 ml per 54 sq. in., which equates to about 0.019 ml per sq. in. Then let us say 10% of your total output ends-up in the waste-basket for whatever reason, you need to divide this consumption by (1-wastage %), which would be (1-0.1) = 0.9, then, (0.019/0.9) = 0.021 ml per sq. in. including cleaning and wastage. Then I would add something on top of that to cover for the lack of certainty about whether future performance will be yet less efficient than current performance.
It remains nonetheless true that in the larger scheme of things, the cost of ink isn't what makes or breaks commercial photography. The value of time and the combined costs of other overheads, such as rent, transportation, equipment and utilities must swamp ink costs by quantums.
