To address this, I performed some fun excel calculations, since our machine has no cost accounting software.

I took 10 posters, 5 with heavy coverage and 5 with light coverage, and printed them. I looked at each print's details in the software to find out the amount of ink used. Added all that up, and averaged it out. So I knew at that point that the average poster of X size would use Y amount of ink. I then did the same with the rolls of paper we keep in stock. If a roll of paper costs X, and 1 posters used Y amount of the roll, I can find out how much paper cost is attributed to each poster. So now that I knew my average mL of ink per linear inch, and my average paper use per poster, I can calculate an average cost with [AvgInkUsePerLinearInch]*[InkPricePer-mL] + [AvgPaperUseLinearInch]*[AvgPaperCostPerLinearInch] =[AveragePrintCostPerLinearInch].

I kept the calculations in excel for two reasons. First, is that I needed to do this with each of my media types, which vary wildly in price. Second, after running more actual production jobs, I could keep pulling data about ink usage and linear inch usage, and updating my excel file for a more accurate average. After 500 paid customer jobs, I ran the numbers again, and was glad to see it was still rather close to my initial estimate of cost. It actually ended up going down because I run a lot more low coverage things than heavy coverage.