I think some of the key variables have been touched on correctly in this discussion, but they aren't easy to separate. To get to correct density on a finished print, i.e., neither too dark nor too light across the full tonal scale and assuming the influence of different media/media settings has been taken out of the loop, different printers do indeed lay down different size drops in different ways. Ultimately, the colorant, either pigment or dye, has to be built up gradually to get to the correct density. For printers with dilute inks such as LC, LM, LLK, LK, etc., the colorant-to-solvent volume ratio is considerably lower, so more ink (mainly comprised of solvent) has to be jetted to build up the image density. Downside is more ink consumed, but upside is smoother midtone and highlight gradations with less noticeable dot structure. How the printer driver/firmware LUT handles the ink channel ramp from a more dilute ink to a less dilute ink, e.g., from LC to C, LM to M, or LLK to LK to K, etc., thus also enters into how much solvent load is going to be used to arrive at the final image density and hence how much ink gets used on average to create any given color. Then too, even simple CMYK printers perform GCR (gray component replacement) to varying degrees, meaning that the black channel can be substituted for ,C, M, and Y coverage when making low chroma colors, and ink channel substitution gets far more sophisticated when considering printers that can substitute red/orange, green, blue/violet inks for Cyan, magenta, and yellow combinations. A red/orange ink for example, can be used substitute to varying degree with magenta and yellow inks, when building skin tone colors, but only if the ink channel algorithm allows it. Thus, how much ink channel substitution occurs to produce any given color depends on the firmware LUTs and software media settings built into the printer by the color scientists who designed the RGB to printer color channel conversion tables for that printer.
What is clear to me from the spreadsheets and from various printer amortization studies I have doing lately, is that none of these printers is capable of competing economically with 4x6 RC photo paper sizes readily obtained from major photofinishing service bureus, yet every single one of them is fully capable of ultimately matching photo lab prices when printing RC photo paper enlargements, especially 8x10 or larger if you make enough prints regularly.
Moreover, if you choose to print on fine art media, say Canson Rag Photographique or Hahnemuhle Photo Rag Pearl, then DIY printing can pay for itself in just a few dozen prints because a custom lab that offers these fine art media is going to charge 10x or more over the cost of printing on inexpensive RC photo paper, but the increased paper price doesn't account for that markup. This conclusion, does of course assume you know how to print in a fully color managed workflow and aren't wasting a lot of ink and paper trying to get the colors and tones where you want them in the final prints. That said, you'd have similar disappointment in print quality handing off your image files to a pro print Lab if you don't know how to prep files correctly for said print provider. Some good pro labs will of course make additional corrections and even handle file prep from start to finish for you, but that's even more expensive.
How fast a home printer gets you to a break even point compared to outsourcing the work is actually more a function of the initial cost of the printer, not the ongoing consumable charges, and of course, how much printing you do. So, an inexpensive inkjet photo printer will reach break even on 8x10 RC photo print pricing in a few hundred prints while a more expensive printer model like the Canon Pro-1000 is going to get there at around 1200-1500 8x10 equivalent size prints. They all get there eventually, but the best advice I can give to anyone thinking of bringing photo printing in house is to have a serious discussion with yourself about how much printing you are likely to do in any given month and how important is it to you to craft your own prints from start to finish. Again, if you intend to routinely print enlargements of 8x10 or bigger on high quality fine art media rather budget RC photo paper, you only need to think in terms of a few dozen prints per year to easily justify the total ongoing costs of printer, ink, and media... printer and media choices playing the more dominant role in total cost of ownership amortized on a cost per print basis, notwithstanding how most endusers obsess about OEM ink prices.
cheers,
Mark
http://www.aardenburg-imaging.com