People are much more willing to give Apple their credit card number than some unknown (to them) web site.

Amazon works the same way. Just yesterday, in fact, I was looking for a replacement part for one of my tools. Google search turned up several options for the part number -- several "parts warehouse"-type sites and Amazon. The price was a couple of bucks higher on Amazon, but I bought it there nonetheless.

Why? Because Amazon already has my CC info, and I trust them to actually deliver the product and keep my information secure.

The other sites may be (in fact, probably are) trustworthy, but I don't know that. I have no idea how well they're securing my card information. I have no idea if they're even going to deliver the part in a timely manner. Amazon wound up getting my money, because saving two bucks just wasn't worth the risk and extra transaction friction.

Same thing with the app stores. The dev is providing the software, and Apple is providing the infrastructure and the reputation.

Is 30% too high for that? Maybe, maybe not. For an unknown dev, it may well be. If you're a well-known company with an established reputation, maybe it isn't. I've noticed that a lot of the big names (e.g., Bare Bones Software) have exited the app store of late.