On some servers Gold Bars are worth a lot.There are 2 types of gold bars:quest and non quest.I saw guide about quest ones and now im posting about non-quest :D.
Make a lvl 30-35 char and do quest The Wishing Potion(guide here http://l2vault.ign.com/View.php?view=Guides.Detail&id=32).
On the end of quest when u choose ur wish, u must choose "i wish i have 100kk adena" or smth like that(involving money).
There's like 90% chance that 3 Grimas(monsters) will spawn and attack you.When you kill them they drop gold bars(almost always).
Thats 3 GBs in half hour.Enjoy!