how much does it cost to put ads on an app?

i need to do my homework for tomorrow and i need to know how much does it cost to put ads on an app.

They are most often provided by ad networks that charge per impression or per click, and are a few cents a click in some cases.

This study find that average cost per 1000 impressions was about $0.75.
www.adweek.com/news/technology/mobi...

This one mentions some considerably higher figures, closer to the typical desktop ad costs
online.wsj.com/article/SB1000087239...

googlemobileads.blogspot.com/

www.google.com/ads/mobile/