WARNING: Create Backup file before doing.
Locate bgruppen.dat file in drive letter:1503 addatedirectory. Use a text program to open it, and search for the following lines
Id: ALKOHOL
BauFirst: INFRA_STUFE_1C
Preis: 900
Bonus: 5, STEPPE|PRAERIE|DSCHUNGEL
PreisLevel: 0,5,10
Change preis accordingly to how much you want, and people will pay that much for each item.