Just replaced an HP 74 black ink cartridge and purchased another backup cartridge for $13.11 (well below the standard price). Difference in weight between the full (28g) and empty cartridge (23g) is 5 grams. Assuming ink has the same density as water, that is $9507 a gallon. Assuming the 200 page per cartridge yield from HP is accurate, that’s also 15.26 cents a page.