It's not really this complicated, you just compare the bulding's cost to production ratios and pick the most favorable one.
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Ah, but then the question is when do you stop buying because not spending for the production boost is more efficient, or if it's more effective to buy a cheaper, less efficient one first so you can afford the more expensive, more efficient one sooner... I dunno, maybe I'm over-complicating things in my head. This just feels like a case where the greedy solution isn't necessarily the optimal one.