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yeah, it would be very expensive too. might as well just get the m-conversion and a helicoid adapter.
Contax G actually has a longer registration distance than Leica M, and there are now a number of Leica M helicoid adapters (some for as little as $35. Personally, I'd pay as much as $90 for such an adapter)
If I did the m-conversion, I'd have to do it for all 3 of my G lenses, which would get pricey.