What is a good CPM?
Perfectly good question!
With no correct answer...
The CPM depends on many things, especially the audience.
If you are selling tractors you would probably not reach the right audience at perezhilton.com. So a CPM at 1 USD here could be a bad CPM for you as an advertiser.
But if could have an ad on the tractor section of eBay you would probably reach the right audience. In that case a CPM at 20 USD could be a good CPM.
If you are selling tractors you would probably not reach the right audience at perezhilton.com.
If you're using re-targeting, behavioral targeting or some other sophisticated targeting you could even accept an even higher CPM.
On the opposite end of the scale you have RON, Ron of network, which is the non-premium and "low value" inventory at the publisher or ad network. They tend to sell this at a very low CPM, instead of being left with unsold inventory.
The size and location of the banner is also important.
CPM at 10 USD for a 300x250 on the top of the page could be bad, while CPM at 5 USD for a 200x200 at the bottom of the page could be even worse.
I also recommended using the CPM calculator to make sure you get your numbers right.