Thanks Jason!!
Added a parenthesis and it worked perfectly.
$WADbSearch1->addComparison("replace(list_price, ',' , '')","".WADS_stripCurrency($_POST["start_price"]) ."","AND",">=",1);
$WADbSearch1->addComparison("replace(list_price, ',' , '')","".WADS_stripCurrency($_POST["end_price"]) ."","AND","<=",1);