Hi Wolfgang,
The total cost of the trip of €108 is correct becuase that is the actual cost of the total trip and this is standard functionality. If you don't want the cost of expenses paid by the company to be added to the total cost then you have to make changes to the travel expense statement (form). There is no where in customizing that you can define whether the paid by company cost cost doesn't get added to the total cost of the trip.
Hope this helps.
Ankur