Why are Leica IIIFs & IIIGs so expensive?

Mainly because they have become collector's items. Despite being over 40 years old, far more awkward to load, focus and use, as well as more expensive to repair than Ms.

Street photographers also snap up late model III's as they are smaller than the M and almost as quiet. Also, compared to modern cameras, they look so unusual and "retro" that they are regarded as being less threatening for candid people photography.

See this May 2004 photo.net discussion on the practical benefits of using a IIIG for everyday shooting:

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