your comparison is for less than or equal, so the NULL Agument of the COALESCE Statement should be something that is less than what the user is searching for, i wouldn't expect the user to be searching 1,000,000, so using 999,999 as the NULL argument isn't very likely, you should use a smaller number like 1
$WADbSearch1->addComparisonFromEdit("PreisWeitere","COALESCE(PreisWeitere,1)","AND","<=",1);