Presence in the app store is not a replacement for traditional marketing. The App Store can get crowded and while someone may find your app because they'll use the search feature, they're not necessarily any more inclined to be looking for your specific app than they would be if you weren't in the app store (and only had your own site).

The huge benefit is you're only competing for eyeballs with other apps for that platform and not a million web sites with terms that may match on searches - when someone is searching directly within the App Store.

Google presence is still going to be important and so too will other sites reviewing and/or pointing to your product.

Paying 10% to process payments is low if you're looking at Kagi or eSellerate. It's bat-shit high if you're looking at a proper merchant account or PayPal (payments standard or even Payments Pro - where a customer would not know at all you were using PayPal for processing).

I suspect Apple must be paying well under 2%.