Interesting thought. Durable rares are, I feel, kinda hard to balance, but I think this does a pretty decent job. At first it seemed a bit overpowered, but then I start comparing it to other rares.
Assuming 3/3/3 facilities, this card doesn't really become worth it until you have about 180 total resources, assuming you want about a 1:2 resource to damage ratio (which is worse than Troll).
Given that amount of resources, distributed evenly, you can win in 6 turns with Bank, in 1 turn with Castle Beyond Looking Glass, in 6 turns with Duplication, and (unless your opponent has a high defense going) probably in 1 turn with a heavy hitter like Spectral Dragon or All Elemental Attack.
Of course, some of those are relying on having an even distribution of resources, which this card doesn't, so that's one advantage.