1. Paid ads - ChatGPT could offer paid listings at the top of its answers, just like Google does when it provides a results page. Not all people will necessarily leave Google/Gemini for future search queries, but some of the money that used to go to Google/Bing could now go to OpenAI.
2. Behavioral targeting based on past ChatGPT queries. If you have been asking about headache remedies, you might see ads for painkillers - both within ChatGPT and as display ads across the web.
3. Affiliate / commission revenue - if you've asked for product recommendations, at least some might be affiliate links.
The revenue from the above likely wouldn't cover all costs based on their current expenditure. But it would help a bit - particularly for monetizing free users.
Plus, I'm sure there will be new advertising models that emerge in time. If an advertiser could say "I can offer $30 per new customer" and let AI figure out how to get them and send a bill, that's very different to someone setting up an ad campaign - which involves everything from audience selection and creative, to bid management and conversion rate optimization.