Hi Costa
System wont allowed you to calculate with fixed qty specified. Instead i would suggest you to do below method.
1) Create a new plan version and do copying of actual cost to plan version using t-code KP98
2) Put your fixed volume activity in the cost centres where you want to calculate the prices.
3) Run activity price calculation
4) Once your actual activity prices are calculated, you can transfer it to 0 version for next month confirmation as per actual prices
Regards
Bhaskar