Looks like the new price, added by the custom field, is not updated in the productpage or checkout page.
I created a custom field option that is 10 Euro more expensive than the standard product. If the customer chooses this option the price should be updated in de product detail page. When added to the cart, the product with the new price is in the cart.
Now the new option is in the cart, but the new price is not! So I still can't use the custom fields.