Good point. Dumb question.
It's not really a dumb question at all. The fact that Plunder isn't available from the start of the game is extremely relevant to its cost. As a standalone Kingdom card pile, I'd probably cost it at $6, and indeed Plunder did cost $6 for a while even under Encampments.
A card that's just +1 Action and +2 VP is bad for the game because it creates a way to infinitely generate VP without pushing the game closer to a conclusion. In fact it creates an incentive to
not end the game, to keep building up a VP lead until you cannot possibly be overtaken. So uh, there's no appropriate cost for such a card.