the question isn't as simple and straightforward as one might think.
although in stores prices on particular cartridges vary by quite a bit, so does the volume of ink they give you in each cartridge. buying the cheapest cartridge might net you the least amount of ink thus resulting in the same $/vol of ink.
two cartridge systems (1 black, 1 color) will be cheaper to use however if you use up one of the 3 primaries in the color cartridge you will have to replace the whole cartridge (unless you got into refilling your own).
multi-color cartridges are easier to replace and will not waste ink if you print color on a regular basis. if you don't print in color regularly (or if you dont print regularly at all) then you have a chance of ink drying up and having to replace all the cartridges anyways.
the cheapest printer to run would be a laser printer (black and white only). great for high volume printing.