"Gerd" wrote:
I have column A with Prices; column B contains formula
- Price x 0.05; column C contains formula - sum of Col A
and Col B. [....]
Is there any way that I can modify the formulas so that
when there is no price in column A that nothing will show
up in columns B and C.

I don't know of an easy way (e.g, a Calculation option).
But you could put the following formulas in col B and C:

B1: =if(isblank(A1),"",0.05*A1)
C1: =if(isblank(A1),"",A1+B1)