I like the card concept, I for one would consider something like this:
4
Treasure room
This card can't be targeted by spells, permanents or abilities (=immaterial)
for 1
target permanent becomes immaterial for 3 turns.
Basically you would have to pay 1 every turn, to protect maximum 3 permanents.Upgraded version:
3
Treasure room
This card can't be targeted by spells, permanents or abilities (=immaterial)
for 1
target permanent becomes immaterial for 5 turns.
Basically you would have to pay 1 every turn, to protect maximum 5 permanents.