What is the easiest most cost effective way to print conductive ink?
Boy, this seems like an HP7475-style plotter hacked to take a pen with conductive ink would be the way to go.  These were plotters that moved pens in x and z, while y was handled by a paper-mover.  I don't know if there's anything like it out there today.  They were pricey in the late '80s.

Poking around a bit, I can find a ref to them being used for PCB

http://www.youtube.com/watch?v=IG9KNDvoRl4