By the way Jan,
by changing your SQL a bit, you may gain some speed:

Select (Sum(Menge*Preis)+Sum(Versicherungspreis)) from Orderd
where (Linknummer='12345') and (retoure='1')
and (NOT EXISTS (Select 1 from Rueckzaehler
where Rueckzaehler.Ordernummer='12345'
and Rueckzaehler.Zaehler=Orderd.Zaehler))

And if any of those numbers should happen to be defined as Integers (or
anything not a string), drop the single quotes.

Set