If you have a huge website, chances are that you have some promotions for some of your own products, sections or sites in advertisements which are often called “house ads”. House ads get lower priority than paid ads from customers and even from content based ads as such as Google Adsense, Yahoo! APT or your own ad system. However if you have to pay for house ads on your own site, you really need to see how many house ads you are running. Let’s say you are running 1,000,000 house ads a month and you’re getting charged $0.50 per CPM (cost per thousand impressions) then that means you are spending $500 a month just on house ads.
Another idea would be to frequency cap your house ads (only show them once per user per day). That would reduce the amount that you show, if you really need to show them.