> Apples Cut/Profit: $30/$29.99
> Dev Cut/Profit: $70/$42
Apple's cut is $30 but their profit certainly isn't $30 minus one cent.
Obviously there's the hard cost of transactions, which includes but certainly isn't limited to merchant fees. If it's an international app, there's the currency exchange involved in selling that app in local currency, under local merchant rules. Apple also absorbs various costs caused by avoiding and dealing with fraud. The transaction could be performed with gift card, which means maintaining that network, physical manufacturing costs, retailer profit margin... I seriously doubt that Apple even breaks even on the 70% when the payment uses a gift card.
There's the work Apple does in policing their store. While it does irritate developers, the end result is that the App Store is significantly less overrun with scams and junk than it would otherwise be—which means that customers are going to be more trusting of your app than would be the case if the store was a total free-for-all.
Then of course there's a subset of consumers who discovered your app because it's in Apple's app store. Providing you these eyeballs is valuable and worth something.
And of course there are a lot of soft costs—Apple did put a lot of work into all the resources for developers. How you personally believe these should be accounted for is obviously a matter of personal opinion, but that work does cost money and has benefited developers. There's an argument to be made that some amount of it could be assigned to Apple's app store revenues.