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