Lihzahrd Power Cell

Every temple generates a chest with a cell already sitting in it, so the first Golem fight costs nothing beyond getting through the door. Everything after that is farming. The residents drop them rarely, so a small enclosed farm somewhere central in the temple pays off faster than wandering the halls, and the same setup feeds you bricks and tablet fragments while it runs. Occasionally that chest generates close enough to the entrance that a determined pre-Hardmode player can reach it.

With spare Temple Keys and a Picksaw, generating fresh worlds and looting their temple chests is faster than killing anything. The cells travel with you. Golem will only answer in a world where Plantera is already dead, though, so bring them home to spend them.

This has the lowest rarity floor of any item in the game. Lava that would burn a Copper Coin destroys a power cell just as readily, and the temple is wired with lava traps. Never drop one on the floor while you are clearing the place, and if you play a mode where death scatters your inventory, treat carrying a full stack through the temple as a real risk.