I guess that most expensive item tittle belong to all "1 of a kind items", (like the mini me that Sentral got duelin') since you can buy your own custom 3d item paying around 50$ (max of 1500polis and considering that 50$ are about 1.000.000TC nowadays) and for a set of items the price is negotiable in $ too, also maybe some event items like the one who Lask used to have "Keap67's Shield".