I'm pretty sure I signed up on Spotifys website, then logged into my iPhone. Apple definitely don't get 30% of that subscription fee.
Because it's forbidden by apple to inform your user that it's cheaper on the website.
A few quotes from the timeline:
> Apple now prohibits buttons or links to any other external ways to pay.
> While we haven’t been able to include any buttons or external links to pages containing product info, discounts, promotions, etc. (even if they don’t link directly to a payment system!) since Feb. 2011