How do advertisers calculate cost per impression?

I’m about to launch a new website and I have to submit my business plan to get more funds. I know how many users I can expect and I know I will make money with banner ads but I have no idea of how to calculate CPI( Cost per impression) or CPM ( Cost per mile). I read on comscore.com that the average CPM for the US was 2,43 USD. When someone says something like, “our CPM is $5” how did he get that number?