[joshvm]

Depends on a large number of things. As an advertiser you can pay based on location on the site (e.g. the advertiser will pay more for a banner than a little side unit below the fold).

Some adverts are pay per view, pay per click, some are commission based (referrer gets a cut). Sometimes it'll be a few dollars for a few thousand views, other times it'll be a few hundred dollars for a week's visibility.

For click throughs, fractions of a cent to a few cents I guess. Commission is obviously worth a lot more (a few percent of the purchase price typically), but harder to convert users.

Have a look at buysellads to get an idea of how this works. Unsurprisingly Adblock has a field day if you try and view the website with it turned on ;)