Sorry to be out of the loop on this. I have an Epson 2200 and haven't really paid much attention to developments with other manufacturers. I gather one is able to buy third-party cartridges for HP and Canon at significant savings? How do HP and Canon ink prices compare to Epson?
That's quite a complicated question and I don't know of any definitive studies done on that. The problem is that ink cartridges vary in size by manufacturer, different printers use different amounts of ink and of course the subject matter of printed material.
When I bought my non-photography printer (Canon Pixma ip3000) Canon had the lowest per-page printing in the ~$100 printer category. Whether that's the case for photography printers is another case altogether.