a ballpark figure = eine ganz grobe Schätzung
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GOOGLE INDEX
ballpark figure: approximately 379,000 Google hits
STATISTICS
IN THE PRESS
When a patient asks me in my office how much her CT scan is going to cost, I can't even give her a ballpark figure
(The New York Times)
Did you know?
ballpark figure noun phrase
An acceptable, roughly accurate approximation
The Free Dictionary
--- All of the figurative uses involving "ballpark" are Americanisms, and refer to a place where baseball games are played. The word is found in British English, but it is a borrowing from American usage.
The main sense of ballpark is "an acceptable, expected, or approximate range". In this sense the word is almost always found in phrases of the sort "in the ballpark" or "out of the ballpark."
A slightly later sense is "one's home territory, regarded as giving an advantage over an opponent; (hence) a field of expertise or knowledge".
Finally, ballpark is used as an adjective meaning "approximate" (a ballpark estimate) - you will often hear this usage in business.