[Anstieg] ... zum Stillstand kommen
has not yet been mentioned as a way of saying "to plateau" in German.
Thus, for
Prices have pretty much plateaued.
write
Der Preisanstieg ist so gut wie* zum Stillstand gekommen.
*so gut wie is equivalent to "pretty much".
Note the difference between zum Stillstand kommen and the previous three answers
(1) sich stabilisieren (2) stagnieren (3) auf hohem Niveau einpendeln
The first two do not state whether prices had been rising (inflation) or declining (deflation), the third one sort of does. Sometimes, though, you want to be more explicit, and then Anstieg plus zum Stillstand kommen can be a good choice. (If prices had been falling, you would use Rückgang plus zum Stillstand kommen instead.)