??? FR-4 expensive? Compared to what?
I hate to be a pest, but a search on Google for "conductive ink" got over 4000 hits. Dow Corning makes conductive inks. Also, you could use some of the metal systems for hybrids, but I can't imagine either approach being cheaper than standard copper on FR-4.
Conductive inks require either a dispenser system, which is slow, which impacts cost, or silk screen. This has to be followed by a cure, which is slow, which impacts cost.
Most conductive pastes and inks cannot tolerate standard reflow soldering, which means you have to use low-temp solder paste, which is slow, requires a longer cure cycle, which impacts cost.
TTFN