Apple official informations : App Store - Principles and Practices - Apple (IE)
You are delivering a digital good so that puts you firmly inside the kind of good to be charged for - how it is generated is utterly irrelevant.
If you have a complete billing system outside of the app and the user can never get to a page with signup or billing information from inside the app, then you can still avoid having to pay 30%.
Evernote are a great example - they provide a web-based login page, which appears inside a view in your app, as part of their SDK.
Apps using that login page were banned because the page allowed people to follow a link to an evernote home page from which they could then get to a billing page.
So Evernote changed the link to being plain text with the evernote . com URL visible and able to be copied but not clickable.
That still was considered a violation by many reviewers. Some apps struggled and appealed with assistance of Evernote corporation.
Eventually, Evernote gave up and you can now buy their premium service with IAP and so Apple get their 30%.
However, Apple don’t take 30% of sales you make outside of IAP. They just demand that you offer the same digital goods for IAP at the same price or lower than you charge on your other store. (ie: you’re not allowed to inflate your IAP prices to cover the 30%).