I'd actually be surprised if Apple pays anything to OpenAI at the moment. Obviously running some Siri requests through ChatGPT (after the user confirms that's what they want to do) is quite expensive for OpenAI, but Apple Intelligence doesn't touch OpenAI servers at all (just Siri has ChatGPT integration).
Even then, there'll obviously still be a lot of requests, but the problem OpenAI has is that they aren't really in a negotiating position. Google owns Android and so most phones default to Gemini, instantly giving them a huge advantage in marketshare. OpenAI doesn't have its own platform, so Apple having the second largest install base of all smartphone operating systems is OpenAI's best chance.
Apple might benefit from OpenAI but OpenAI needs Apple way more than the other way around. Apple Intelligence runs perfectly fine (I mean, as "perfectly fine" as it currently does) without OpenAI, the only functionality users would lose is the option to redirect "complex" Siri requests to ChatGPT.
In fact, I wouldn't be surprised if OpenAI actually pays Apple for the integration, just like Google pays Apple a hefty sum to be the default search engine for Safari.