When you buy an app or in-app purchase from the App Store, Apple takes a 30 percent cut of the revenue. This applies to subscription fee when you sign up in-app for web services such as Spotify. Instead of paying its cut of revenue to Apple, Spotify chooses to pass that cost directly to consumers. As such you will pay higher fee when signing up from within Spotify for iOS app than on the website.
As reported by ArsTechica, YouTube’s newly announced ad-free subscription service YouTube Red is taking the same approach. It is cheaper to signup YouTube Red from its web page at www.youtube.com/red:
There is a big catch about that $9.99 price: $9.99 will cover Android, desktop, and the mobile Web, but if you purchase a subscription via Apple’s in-app purchasing on iOS, the price goes up to $12.99/month. Apple takes a 30 percent cut of all subscription revenue on its platform, and Google is passing that cost directly onto the consumer.
As a side note, you might notice that some apps such as Dropbox, does not offer you to subscribe for its paying plan directly from within the iOS app. As per Apple App Store policy, if an app does not offer in-app purchase of its subscription plan, the app can not advertise such plans or provide a link to its webpage from within the app.
Not many web services are passing the 30% cost to consumers. Before you subscribe to any web service paying plan, check and compare the prices from within iOS app and on the website.