Go to the 'bgruppen.dat' file in the 1503 ad\data\ directory. Open the file in notepad and look for this:

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

Now edit the preis line to whatever you want and your people will pay that much as the price for that particular item. You can change the prices of other lines also.