Depends on if they pay per view or per click. And then depends on the site/app, the location of the ad, and the bid.
For the Play Store, the CPM is usually around $10-15. So for every 1,000 people who see the ad, they pay $10-15.
YouTube can be $30-40 for the video ads with some going up close to $80. An ad at the bottom of a news site can be as little as $0.50.
The ads you’ll be shown will be the highest bidder who meets the criteria to display it. In general, the more targeted it is, the higher the bid. So McDonald’s might just say, “show this ad anywhere on the site to anyone who is in the US” and bid $0.25. A high-end watch company might say “Show this at a banner ad to men who are between 30-45, have an income above $150K, who care about fine products, likely own multiple nice watches, and are visiting high end watch websites” and bid $50.