Most sites in the top 10k are able to name their prices, and they charge on an impressions basis, though they may market it as 'a week long run', 'month long run', etc. What advertisers are concerned about is impressions and unique visitors. The larger the site, the more they are able to set their own price.
Daily Kos, to pick an example, charges $1k per day for a single, rotating, ad run, in a similar market. They have several ad programs, and are roughly 5% of HuffPo's size, and go through BlogAds. They look to be earning about $3m/year in revenue. This is also roughly comparable to their size differential versus Salon.
Huffington Post probably charges $20k+/day for a single, full-site, prominent ad.
For adult sites, this gets turned completely on its head. The industry is littered with fraud, scams, and other problems. Google won't touch us, Yahoo won't touch us. My options are basically restricted to extremely shady companies, affiliate programs, or pennies that I will most likely never see. Moot breaks even with the penny route.
So I've gone the affiliate route. It makes a dollar a day. Other sites in my size range do 10-20 times that, but they aren't adult.