Actually "opportunity cost" is one of the few concepts I'm grateful to formal economics study for. It's not the same as bird in hand. I can't think of any colloquial equivalent for it.
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This gap in colloquial language has been bothering me, and it seems to me that a list so inventive in toponyms ought to be able to correct it. I thought at first that "Cast your bread upon the waters" or references to "eating your seed corn" would pass as opportunity cost colloquialisms, but they're really more about consumption vs. investment, and I don't think it's the same thing. I was trying to invent a fake colloquialism, like "Eat steak now, no more milk from the cow" or "Grape juice is fine, but it's future wine," when I realized the problem. So if some of the toponym hounds could divert some attention to this much greater need . . . .
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