I am on about CPM networks and not google's cpm mechanism. It would be helpful if you can provide details on the amount of traffic you get every month, your earnings from CPM network, and the network you are using.
Some CPM networks pay around $0.40 - $1 cpm, so with these networks you can see how much people are roughly earning by how much they pay.
CPM networks don't pay a lot anymore, at least not like "the good ol days". You need a lot of traffic to make money off them. I'm talking 1000s of impressions a day; preferably 10s of 1000s. If you're not doing that kind of traffic, I'm not sure if it'll be worth it, best to stick to PPC and not litter your site with banners.
It depends. I have a section on my site where I earn more from CPM than what I do from CPC. So I only use CPM ads on these pages. Then there are also other pages on my site where I earn more per sale, than what I do per click, so I then don't have any CPM or CPC ads on these pages and just have the pay per sale ads instead. It all depends on your sites topic. Some topics you will earn more from CPM than what you will CPC.