Whoever deleted my comment this is an EN section.
The rules say that you must write in English. Those who try "help" (including me), at least must understand what they are reading. So instead of deleting comment, junk the topic or write to this guy that this is an EU topic.
Fix me please if i'm wrong else we all start writing in our mother tongue anywhere.
You open more topics within a week than i ever seen.
Why don't you hire a developer to make these features for you? This is help section no request section. I said that because help mean you post some code or what you did so far at least to confirm that you're trying to solve.
Nonetheless the file you're looking is called RequestBuyItem.java and the 2 variables that affecting the price of a buylist are those two:
double castleTaxRate = 0;
double baseTaxRate = 0;