Well, aftermarket gauges tend to be expensive because of quality. Take a look at the speed hut gauges for instance. THOSE are some nice, high quality, SLIM gauges. Well worth the near 800 dollars for a 5 gauge set.
Anyways, I think the only way you would be able to use that V12 gauge on a 6 cylinder, is to make some sort of special circuitry that can take the original tach signal, and boost it to 12 signals instead of just 6. I have no idea how it would be done, or if it would be possible however.