Ignore my earlier post. Try this -
1. Create new formula with just parameter SAP_55 in calculation.
2. Then in the work center costing tab go to the Formula Constants and assign value to parameter SAP_55 and assign new formula to activity type.
Now if you create new order cost will be calculated based on value assigned to SAP_55 only.
I tried this on my system and it gave me same cost no matter what work or duration was assigned to operation. Of course I only checked planned cost but I think it will work for actual cost also.
Aaditya Palsule