Forum Moderators: skibum
Let's say a site has 5K visitors per day and earns $$$ per day with a combination of CPC and CPA. Theoretically, it could squeeze in a CPM banner, but it would not be worth it. Net CPM is often under $1, so the site would only earn an extra $10-$15 per day, and the CPM ads could 'steal' several times that amount from the CPC and CPA ads. Remember, CPM ads get down and dirty to maximize the click rate. They could steal clicks from the CPC ads and credibility (conversion) from the CPA ads.
Now, having said all that, CPM ads, at least from the major networks, could at least cover bandwidth until you find better alternatives. Just do not get complacent earning fractions of pennies for lookers. Try whenever possible to get quarters for clickers or dollars for buyers. Test, test, test until you find what works best for your site.
FYI - general networks include doubleclick, fastclick, burstmedia, tribalfusion... you can also find a category for "Advertising, Banners, and Exchanges" at ODP and/or the Google Directory.