Sites typically follow a CPM model ... cost per 1,000 impressions. If you charge $30/CPM, you'd make $30 every 1000 impressions of the advertisers ad.
CPMS vary on industry and how targeted your site is. The better targeted group, the higher the CPM's. For example, if you own a Single Mom's website with 5,000,000 page views, you could charge a very high CPM vs if you owned a generic news site with similar page views.
Or, you can charge a click-thru fee. They pay you everytime your users click on the ad.
Our site will be specific, and there are several industries that I would think would want to advertise on our site. ...How would you figure a click thru fee, and it one preferred over the other by people looking to advertise? ...What are impressions?