Find the "config.txt" file in the main game
directory and open it. Here you can change 
the costs of all products. 

Furthermore, by finding the following line, 
you can change the amount of money you start
with at respectively the easy, medium, and 
hard difficulties : 

startkapital(1) = 6000000 ;einfach 
startkapital(2) = 4000000 ; mittel 
startkapital(3) = 2000000 ; schwer