Yes, but remember that retailers can (and often do) promise to match the best price that you can find at their competitors. This is because retailers are in competition with each other for the same products.

This analogy does not apply to Apple, because it has no motivation to price match, because nobody is allowed to sell i-device apps except for Apple.

Edit: in addition, if we follow your analogy further, Costco could demand 30% from Apple on all purchases made through the App Store, for iPhones sold in Costco. Just like Apple demands a cut of all purchases after they sell a product in their store.