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

To get free items, look for the following words in the file 
and set the values in front of them to "0":

kaufkosten
monatkosten