FWIW, the quote about money on the ground initially comes from a critique on the efficient market hypothesis, not commentary on the nature of ideas. So it's expected for there to be analogical disconnect here.
For it to work as intended, you'd have to construct (or buy into) some framework for an "efficient marketplace of ideas."
Absolutely and I am aware of its origins, but I've seen the analogy brought out at least a few times here on HN when discussing the merits and potential of an idea that seems obvious but has gone unexplored or uncommercialized. I get the sense that there may be some beliefs around the evaluating of ideas, at least in this crowd, that lead some to see it as being comparable to noticing a sum of money on the ground. I'd love to understand that.
Furthermore, I've seen it used in the context of hidden transaction costs in the markets. For example, you see security A trading at $100 on exchange X but $105 on exchange Y. You think "free money!" but on closer examination exchange Y charges $5.01 to transact. These are totally nonsensical numbers, but they do extend to the analogy on ideas.
For it to work as intended, you'd have to construct (or buy into) some framework for an "efficient marketplace of ideas."