"One ad unit placed on top of the website earns more than 3 ads in the footer." Is this true for CPM networks?? I found it odd if the ad pays for impressions(Not clicks!) it shouldn't matter where it is, or does it?
It should not have mattered, but sadly, it does matter in today's online advertisement world. Most of the CPM ad networks are not purely CPM based. Almost all of them follow eCPM model - which depends not only on impressions, but on click/conversions as well. A banner ad placed on top of a webpage tends to generate more actions (click/conversion) than 2/3 banners placed on the footer of the same website. Hence it is often being asked by ad networks to place the banners above the fold. Having said that there is no hard and fast formula - you would be the best judge on your traffic and inventory. So my suggestion would be to test different banner ads at different positions on your site for close to a week and then have a look at their performance closely. Then you would be able to figure out at what position a particular ad unit has performed the best.
it does now a days advertising companies like google are very smart, if conversion is good then they show better paying advertisers and earning increase on those areas
I would find that about right, ads on footer hardly earn any revenue for me unless your site has a large amount of traffic.
I strongly agree with What Debapriya says. try to imagine it from advertiser's perspective, if u were paying to show your ads. would u pay for impression which are not seen by most of the Viewers ? Of course not. The more clicks/conversion means better traffic and hence eCPM for such spaces increase gradually. results may very for different spots and network. Best way to do is split testing.
As a publisher you can't really work with only 1 ad network. And with only 3 ad spots on your pages if these networks don't have a "support tag" feature, where can find room for 6 ads on a page without looking ridiculously crowded?