Well, it does vary by the print of course, but on my 3880 this is where I am netting out on average:
I like to make 13x19 borderless prints and it looks like a full set of inks will make about 228 of these based on my average ink use.
When I make 17x22 borderless prints it looks like I am averaging about 150 of them.
With that said, it seems my paper cost is always higher than my ink cost on these prints.
Also, depending on how often you use the printer, you may have to do nozzle cleaning cycles which does use a lot of ink. And of course if you are frequently switching between matte and glossy printing this will use significant amounts of ink as well.
I love printing at home on my 3880 and will never go back to sending out prints. But I would hardly argue that it is saving me money this way. I just reprinted a few canvas's because I wasn't happy with how I processed them first time around and knew I could redo it quickly. Probably would have just left them if I sent them out but I like tweaking to get things perfect and the 3880 is great for me in this respect.