[WhYdOyOuThInKtHaThApPeNeD]

I second Loreangelicus's recommendation. Never waste precious crystals on suppression. I've used that cape a lot, one of my favorite items to find. Get the Diploma in Anti-Magic from the middle building when you first arrive at Elon (sorry I forget proper names), do the defense test to get that item. When you wear the Magician's Cape with the Diploma in Anti-Magic your army becomes nearly invulnerable to magic attacks, especially those units with already high magic resistance. Even more so if you have the 3rd level Guardian Angel Medal. Makes for super easy item guardian suppression and upgrade battles since they are largely magic battles.

Magician's Cape: +30% magic resistance
Diploma in Anti-Magic: +25% magic resistance
Mirror Defender Shield: +20% magic resistance
Crystal Sphere (gift bag): +5% resistance to all types
Guardian Angel Medal level 3: +7% resistance to all types

For a total of 87% magic resistance + whatever your monsters have endogenously.

Just wait till it reaches 0 morale and fight the keepers for a bit of extra experience which is finite per game anyway. I think the crystal spend is just to add dimension to the game items and perhaps for those who aren't strong enough to suppress it.

When the item reaches 0 morale it will just stop giving you any special bonus until you suppress it by fighting the keepers or giving it crystals. You can keep wearing it when the morale is 0 but it but it's no different than wearing nothing.

The item isn't part of a set. It starts off as Magician's Cowl, after upgrading the 1st time it becomes Magician's Cloak, then after the 2nd and final upgrade it becomes the much prized Magician's Cape