Whether you like it or not, the
fee is 0%.
Regarding the other costs:
> What about the overhead of hosting the site?
Hosting a static bag of bytes (e.g. a mobile app) costs nearly nothing. Just the cost of apple developer program (99$/year) would cover a huge amount of downloads.
> The cost to develop it?
Those are the same when developing an iOS app, except that with the web you would be able to target a much bigger market... if Safari was not actively slowing down the web.
> Drive traffic to it?
Breaking news, you also have to drive traffic to your app.
> Credit card processing fees? Managing fraud on payments?
Other people mentioned 2.9% on stripe. And if you are feeling adventurous, nothing spots you from creating your own payment system.