Spotify: $ to build the app, $ to maintain the music label relationships, $ to manage paying royalties, $ to manage server infra, $ for bandwidth, $ to Apple for the "privilege" of being listed on the App Store and using Apple's payments system (which they have no choice over).
Both companies have the exact same base costs to operate their service. But Apple gets to charge for subscriptions through the app for free, whereas Spotify does not. That's literally the only difference cost-wise, and IMO Spotify is correct to consider that anti-competitive. Whether or not Apple would make more or less money shuttering Apple Music and transferring all their customers to Spotify is completely irrelevant.
And no, I don't consider "creating and maintaining an iOS payments system" to be a part of Apple's costs here. That would exist even if Apple Music did not, and the marginal cost of having Apple Music use that system is virtually nil, certainly not 30% of Spotify's subscription fee.