A ballpark estimate is a very rough approximation of a number,
figure, or calculation.
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A ballpark figure is an "educated guess" (or experienced guesstimate).
Example: I might guess notebooks cost 4-5 dollars each (not on sale). I need 2 notebooks, 1 package of pens, and a new backpack. I guess it will cost around $40.00 as a ballpark figure. I actually spend $39.95 US, and bought a package of pencils too.
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A ballpark is a park in which a ball game, particularly
baseball, is played, or a general vicinity - a broad approximation
or a figure at which something is estimated.
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several are: close, almost, about, ballpark figure, circa.
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This means estimate the figure. Even a large estimate will
do.
I think when most people ask for a ball park figure they are
wanting a realistic price range but recognise that figures will
vary to some degree.