A better analogy might be to ask how much Wal-Mart marks up software in their store. The answer is more than Apple does. Apple's value proposition is they bring x hundred million paying customers with credit cards ready to go into the store and you will sell at least 30% more than if they didn't bring those customers to you.
Is Wal-Mart's markup still bigger than Apple's once you take out the incremental costs of dealing with physical boxes in stores [1]? Considering the substantial markups we see on things like books and DVDs, I suspect not.
[1] To be precise we should also take out Apple's incremental (not fixed) costs for delivering apps, but I suspect those are negligible on a per-app basis.