That isn't to bad of a price for it and it is well worth the money!
If your still stuck on that Motor Ball guy, then you'll find the magic Bolt very effective against him.
Whoever you equip this to you should let them keep on casting it each time its there go, while letting your other two members concentrate in attacking him and healing the group.
Oh and here's a tip when you fight this boss its always a Back attack but you can avoid this by pressing L1 and R1 which you use to escape battle, well if you quickly tap them and let go again before he attacks you at the beginning, your characters will turn round and should avoid the extra damage from a back attack (you can do this trick when you ever have a back attack from a monster)