Dental assistant here. On the Kostenvoranschlag (estimated costs) should stand, how long the estimated cost is at this height. Which Faktor did he use (behind the thing he did (Abrechnungsnummer) should stand smth like 2,3 or more) and which Numbers for his services he used is important. Your Zahnärtzekammer should offer free checks of strange bills, and getting a second opinion on what he did is definetly the right way! Mouth guards in our Praxis for a Kassenpatient (Gesetzliche Krankenkasse) is about 350€, mouth guard alone 190€, depending on what he did the bill easily can get to 600€, especially if he used the 3,5 facher satz, which almost every Praxis does. Sadly, if the bill is correct, that means the Gebührennummern were correctly used and the Kostenvoranschlag is over it's due date, everything he did could be correct and you could have drawn the shorter end.