I'll throw out the question and then give my thoughts about a potential answer, which I am not convinvced are correct.

Where does the phrase "right on the money" or "on the money" come from? (I thought I was wrong after being second-guessed, but it turned out my answer was right on the money.)

The phrase has an equivalent meaning to "on the mark," "on target," "on the spot," &c. My intial thought was that this phrase comes from horse racing vernacular. When a horse finishes in a position high enough to provide a pay-out to a bettor, it said to have "run in the money." Similarly, when the horse does not place high enough, it said to have run "out of the money." These make sense, but they are not exactly the same as "on the money." Similarly, "on the mark," "on target," and "on the spot," seem to be related to target shooting, archery, artillery or some such activity where these phrases have a certain logic. But, no one (that I know) uses money as a target for firing projectiles at. I guess, when it comes down to it, it's the "on" in the phrase which throws me. Does anyone have a guess as to this phrase's provenance? Is this phrase even used in Britain, or is it purely American?
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Thank you Marius for sharing that, but I think the site simply confirms my conjectures on where this might come from. It was gratfying to find that at least one other person had the same reasoning I had in trying to explain the origin of this phrase, but I repeat again that I have little faith in my interpretation as it is based on nothing. The link does not seem to provide anything determinative. Perhaps a coin was affixed to an archery target in olden days? Can anyone confirm this?
On the money comes from cricket. Unlike baseball, in cricket, the bowler (pitcher) bounces the ball off the ground. The ball is harder to hit if it is pitched at a certain distance from the batsmen called the good length. Bowlers would practice hitting the good length by dropping a coin at the good length spot and trying to hit the coin, hence the phrase on the money.

That is entirely plausible.

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BoilerkarlPerhaps a coin was affixed to an archery target in olden days? Can anyone confirm this?

No, and I doubt it ever was because traditionally, the centre of an archery target was well-marked anyway. However, that centre spot, besides being known as the Bull, is also known as the Gold, because that's the colour it was painted. The Gold... the money, perhaps.

Nobody, it seems, has the definitive answer to this. I would like to offer my own suggestion. Auctioneering goes way back into the past. Well beyond the settlement of North America, and cricket, and surveying, British auctioneers were busy selling wholesale crops and livestock on market days, and other things, too. The auctioneer would, and continues to, give an estimated price to the seller and potential buyers for each lot. The estimate, is usually a range, say, £50-£100, which the lot is expected to realize. If the hammer price in this example is £80, the sale is “right on the money”, i.e., it has fetched what was expected. Were the lot to fetch less than the estimate, that would be “disappointing”, but if over the estimate, the seller would be “in the money” and in the mood for celebration in the Market Tavern.

working as a surveyors' assistant It was explained to me thus: When the earliest surveyors do their work, they install "benchmarks", a 1" X 1" steel rod hammered into the ground at a known Location & elevation. Over time the top of these rods would tarnish an be hard to see in the viewfinder of a surveyor's transit. Putting a shiny coin on the top would render them visible & when the transit was sitting level precisely above it, you would be "right on the money".

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