100g worth to cast a concentration spell that only lasts an hour, and doesn't protect against magical attacks... seems under-powered to have such a high cost to me?

I'm playing a Goliath Way of the Four Elements monk and this spell seems to be a really themed choice to take as "Eternal Mountain Defense", however I could take this only one level before I can use KI to turn invisible and since resistances doesn't seem to stack...Aside from the duration, what's the point of  this spell for a monk anyway? I guess I coudl take it, shirt rest and still have 30 mins or so before it runs out...

For some clarification, an ancient dragon's melee attacks are not magical, though the elemental damage would be unaffected. Giant's do not have magical attacks. A Death Knight's melee attacks are not magical. The Tarrasques' melee attacks are not magical.  I mean, 100gp to have resistance to all the Tarrasques melee attacks? The spell is perfectly fine, and like many spells, very situational. Against some creatures, it won't do much to help. Against others, it can turn the tide of battle.