From the consumer's point of view, "free" is the best price. For anything. For any justification they can conjure.
This is classic market economics. The seller offers a product at a price. If people don't buy it, the seller can decide whether to adjust the price in order to attract more buyers. If a product tops the sales charts, there's not much evidence to support a price cut.
We could be talking hardware, software, cars, the daily newspaper... Outside of periodic sales promotions, the price remains the same until the next model/edition arrives. If there's excess inventory of the old model, then they may have an inventory reduction sale before the new model/edition arrives. If there are no inventory problems, there are no discounts.
As far as Aperture is concerned, Apple has zero inventory, hence nothing to liquidate. They may have a motive to recover their R&D costs, but those were probably recovered a long time ago. At this point, every sale is gravy. The only questions that remain are, "Do we want to encourage people to switch to Aperture, when Photos is just around the corner?" and "Will we sell enough at the discounted price that we make more than if we'd sold it at full price to fewer people?"