Here we go. When calculating cost of the HP plotters there are two main costs associated with these systems. Paper and Ink.
The ink is liquid ink the same type used in ink jet printers. I have calculated and done some very good guessing and have arrived at the cost of .07 cents per square foot for the cost of ink on an average HP plotter.
Based on a "d" size drawing which is 24x36 or 6 sqaure feet. The cost for ink is about .42 cents give or take for more or less coverage. I think the cost is higher, but I can not prove it, so I believe I am being conservative at the .07 cents per square foot.
We then have the cost for paper. please remember that the best reproduction on an ink jet plotter with ink jet paper, the same as most ink jet printers. Most companies buy a better grade of paper for thier "black line drawings" (blueprints) and then will buy something cheaper for thier "check plots" (check prints).
The better the paper the higher the cost. I use the cost of .055 per square for paper. Thus with a "d" size drawing the cost for paper is .33 cents.
The total cost per "d" size is the roughly .75 cents per plot (print).
Our retail cost with paper, toner, developer and service is about .063 per square foot or .38 cents per "d" size drawing.
The difference is (rounding) .40 cents per plot (print). This is what the customer will save.
300 plots per month will represent a savings or ROI of $120 per month to the customer.
600 plots = $240
900 plots = $360
Your best way to figure out the customers volume is to ask how many rolls of paper they use per month. Typically the rolls of paper for the plotter come in 150 foot lengths. 150 feet will get the customer 75 "D" size plots.
So in order to have a savings of $120, the customer needs to be using 4 rolls per month.
There are many other savings for the customer, such as scan2file, stopping the outsourcing of copied prints and the time factor to reproduce those plots.
Have to go, hope this helps.