Hi, thanks for your great question!
CPM stands for cost per mille and is an objective to select when advertsing on the Google Display Network. It basically means cost per 1,000 impressions. 
According to TopDraw the average CPM is $3.12, so its pretty affordable. 
You can choose this option if your objective is awarness of your campaign or product to understand exactly how many people have seen your ad. 

Many companies prefer to select cost per click or CPC as you are only charged if someone clicks on your ad. This means people will still see your ad but you only pay for the people that click.  The average CPC is $0.67.

It will depend if you want to know exactly how many people have seen your ad or you care more about clicks to view your website or ad when choosing your objective. 

Hope this helps!