If probably costs Apple a couple thousand dollars on administrative minutia per year. So, ROI is on the order of magnitude of 1 billion percent. Of course, that plummets to an order of magnitude of only 100,000% once expensive lawyers get involved. IOW, the numbers here are crazy.
Your observation that you use GPT is the key one here. We are in the process of having search engines replaced by a new technology. Even if we grant that Google has had a monopoly by dint of this deal with Apple -- which I do not -- that advantage is now being eliminated by market competition from AI.
This is a key issue with antitrust in the age of exponential technology growth. By the time governments identify, build, and prosecute a case against a monopoly, the conditions of the market may have changed to the point that the intervention is either unnecessary or ineffective. The government can only act after a monopoly has been established for a long enough period of time that it doesn’t appear to be just a temporary fluctuation in the market. Nowadays, due to the pace of growth, by the time the government can act and prosecute, technology has already advanced to a point that the monopoly is being broken by competition from innovative competitors.
We've seen this multiple times now. AI started to replace search engines even while the DoJ argued against search-engine monopolies. Internet Explorer had already been eclipsed by Firefox, with Chrome on the rise by the time the EU forced Microsoft to allow a different default browser, not to mention that desktop browsers were rapidly being eclipsed by mobile-computing browsers. In more recent years, Facebook was seen to be a rising monopoly in social media, particularly after its acquisition of Instagram and WhatsApp. By the time the government started considering action to break up Facebook, TikTok was already disrupting the market.
Ultimately, antitrust enforcement in the digital age risks being either too late to matter or unnecessary altogether. If legal action cannot match the speed of technological change, market forces become the only truly effective check on monopoly power.