The cartridges on my company's HP Deskjet 5000 are 580 mL each. The plotter has 6 cartridges in black, yellow, cyan, light cyan, magenta, and light magenta. Depending on what you are printing and the quality, you are looking at something between .1 mL and 1.5 mL per square foot. A paper company around the corner gave us 600x3 ft. of paper for $50. 100x3 feet of photo paper was $128.
Once you have a working plotter, the cost in materials is ridiculously cheap to the tune of between $0.16 to $0.75 per square foot. The printing companies are charging for their equipment and labor. We still use HP cartridges and print heads which are expensive $125-$180. At some point we will be forced to move to aftermarket parts, but I imagine most people are going there quickly based on cost.