Seo is better if you can do that. But people want's to increase their revenue in adsense or any other network i think therefore they buy it.
Once again.. It all boils down to earning & revenue. They pay for traffic as an investment. The more you invest in the correct way , the more your chances are to build up new business. Everyone us after revenue, money makes money & purchased traffic will ultimately leave you behind recurring traffic.
I totally agree with you people buy traffic just because they could get some revenue via cpm sites but when buying traffic people must keep in their minds that when buying this amount of traffic how much will i get in return eg traffic company asks for 1$ for 1k visits and your cpm pays 3$ then this is a good investment but traffic company asks for 1$ for 1k visits and your cpm pays only .5$ then this is not a good investment
Naturally. Your calculations are correct. But its not limited to CPMs. Good traffic can leave you recurring visitors that bookmark your website and come back later. You can even consider CPC's.