Ball park figure

Though this is a culture-specific idiom (USA, baseball), it is today well-known, particularly in the business world. It means a rough figure or estimate, an educated guess within acceptable bounds.

Etymology:

An allusion to the relatively large field in the game of baseball and to the fact that baseballs are seldom hit out of the ballpark, but instead land anywhere within it.

Example:
“I'd say it will cost 500 dollars, but that’s a ballpark figure.”

German translation of “ballpark figure/ballpark estimate”:
Hausnummer, ungefähre Zahl, grobe Schätzung