As I did with my first 7600, I kept very accurate tracking information on ink consumption for the 7800 for first 2 months of use. Factoring waste for head cleanings the ink consumption tracked was 0.021 ml per square inch. So, for example, an 8 x 10 print area = 80 sq. in x .021 = 1.68 ml ink. My ink cost factor is $0.41 per ml (usd) (based upon $90.00 per 220 ml cart) for a total ink cost of $ 0.69 . That number is based upon 1,440 dpi print mode on smooth coated stocks. You mileage may vary.
I hope this helps,
A square foot with an HP Z3100 on matte paper should ask for 1 ml and the price approx. 0.37 $ per ml. Twin packs = 260 ml. Based on European ink price, I have no US $ price for the twin packs.
There has been a thread on its ink use:
Gloss papers will use about 1.5 X that number, with the gloss enhancer added 2X. Gloss enhancer is slightly cheaper and should be set against additional varnishing if an inkset doesn't deliver a good gloss.