man, you want to fix the price you sell the items, or the price you buy them? if you want price you buy them, and gmshop is multisell go to gameserver/data/multisell and find the correct xml of the item you want. then at ingredient value, add the value you want to buy them. make sure ingredient id is 57
if you want the price that sells, go to navicat>l2jdb> then if you want to edit the price of a weapon, find weapon table, of armor find armor table of other item, find etcitem table. then find line price and add how much you want.
here is a little sql mod, to make all items sell for 0 adena:
update armor SET price = 0 where item_id between 1 and 99999;
update weapon SET price = 0 where item_id between 1 and 99999;
update etcitem SET price = 0 where item_id between 1 and 99999;
NOT SURE IF IT WORKS 100%