"Elerium collection from a base defence mission can only happen if elerium pods are generated in the battlescape. If you have 10 units in storage, the game will generate 10 pods in the base, given sufficient space in the object table. But because the way the game automatically converts 1 pod into 50 units at the end of the mission, you're basically getting 5000% more elerium from your own pods (those that don't get destroyed, anyway). None of them come from the aliens! This will only happen in earlier versions of the game. "