Top websites are charging $1.5 cpm (per thousand impressions)
If you are new, you may start with charging $0.5 cpm.
If you are getting 100,000 page views per month it will mean $50 for the advertisement. If you can get four advertisements you can get $200 and if you can manage more, you will get more.
If you join ad networks like adsense etc., you may get less than that as there are commissions involved.
Read many more details in Problogger post