Brian, I'm not sure what you're going at. Lead, of coure, would be cheaper and easier to work with, but it's a poor material and doesn't patinate nicely. The addition of gold to copper gives a nicer patination/finish (but, although softer than copper, wouldn't make the alloy remarkably easier to work with since the amount of soft gold isn't big enough; that's why I only mentioned copper in my last post when talking about an "easy" material). So we're basically looking at a compromise: which material is relatively easy to work with, looks good, is strong enough for the purpose but isn't too expensive? Well, that's Shakudô. The same reason why the vast majority of (non-Tsuba) fittings are made of this material. The same reason why many people buy a Toyota (Shakudô), but not a Porsche (pure gold) or Yugo (lead).
The reason why virtually all Habaki are made of copper is probably work hardening; a Habaki has to be more sturdy than other fittings. I've seen solid gold or silver used for Habaki, but usually those materials are used for plating/foiling because of the look they give; pimping up for relatively little cost.