Which is the expensive bit? The plane I usually fly already has a 'remote indicating compass' (rather an old presentation, vertical card like a DI but the needle revolves instead of the card, so you 'fly the needle' to take up a new heading. It suffers from dip but does seem more accurate than the one on the coaming. Is this using a flux-gate? If so, is the HSI or RMI then quite cheap to add?
Tim