Meanwhile, I have found a solution for the display of the feld 'price' in the list form. In my template a have changed
<? Php echo $ cck_renderFiled ('price');> to
<php echo number_format ($ CCK?> GetValue (prijs_eb), 2, ",", ".");? >.
For my list, it's this. For now I still need something for the input in the admin form.