Use a text editor to edit the "bgruppen.dat" file in the "\1503 ad\data" 
folder. Look for the following entries in the file, about one fifth of the
way down. 

Id         : ALKOHOL
BauFirst   : INFRA_STUFE_1C
Preis      : 900
Bonus      : 5, STEPPE|PRAERIE|DSCHUNGEL
PreisLevel : 0,5,10 

Change the price ("preis"), as already done, to "900" and your people will
 pay it. Obviously, other prices can be changed ("Salz" is salt, etc.), but
be careful. Do not mark up something that you plan on importing.