Estimate pages from a cartridge yield.
Cartridge yield is volume divided by consumption per page, with one correction that explains most of the gap against advertised figures: inkjets spend ink on nozzle cleaning cycles, typically ten to fifteen percent of the cartridge and more if you print infrequently. Removing that first gives usable volume, and a typical text page consumes around thirty microlitres.
Usable volume
Usable microlitres = cartridge ml x 1000 x (1 - maintenance waste)
Yield and cost
Pages = usable microlitres / ink per page; cost per page = cartridge cost / pages
Because cleaning cycles run on a schedule and on power-up, not per page. A printer used once a month can spend a large share of its cartridge maintaining nozzles rather than printing.
Toner is measured in grams rather than millilitres and has no drying or cleaning waste, which is why laser cost per page for text is usually a fraction of inkjet for the same volume.