Where can i change the sell price?

Go to lua/zmlab/sh/zmlab_config.lua

SellRanks defines how much money the player gets per sold kilogramm of meth.

You can add new ranks or change the exising ones but DO NOT remove the "default" rank since it gets used for any rank which is not defined.

For Police you can define a PoliceCut which can be a value from 0-2 for examble.

1 = 100%, 0.5 = 50%, 0 = 0%

The police cut gets calculated by taking the PolicePlayers Rank and getting the sell value from the SellRanks defined in the methbuyer config and then multiplying it by the PoliceCut value.