Three perfect gems of any type man (+ the Grand Charm). Won't guarantee you a BETTER Grand Charm, but it'll re-roll one of the same ilvl as the one you use to Transmute. So naturally its best to use Grand Charms from like, Act 4 Nightmare and above.
the best one that no one seems to know about..... because they're always complaining about repair costs..... is a ral and item in cube repairs item. pretty good for repairing forts and the like which almost cost as much to repair as the maximum amount of money i can carry