I'm not going to get into the exact economics, but Ars writers write about a piece a day, and the average piece gets tens of thousands of visitors. Revenue per article (for most mainstream news site, not just Ars) is significantly better than $0.001 per visitor. High-end advertisers are willing to pay a premium to put their ads next to high-quality content and to know the demographics of the people seeing their ads. None of us is getting rich doing this though!
We're also fortunate to have a significant base of paying subscribers, which makes it easier for us to write in-depth pieces that might not generate enough revenue from ads alone.