The price per aircraft generally decreases the more that you order.
That being said, after the initial purchase of the -300ERs, a very highly placed (non-pilot) manager involved in the deal accidentally let it slip that Boeing had told CX that, despite the original order only being for 36 (16 + 20 options, but please correct me if I'm wrong), they were so confident that we would eventually end up getting about 75 of them, including some freighters, that they gave us the price as if we'd ordered 75, and told us that the "75 order price" would be the charge for any more that we ordered.