Do ads cost you money?

So I would like to add ads to my game. I am using cocoon js and plan on using mopub and admob.

Are there any costs at all for me? Or are there no risks?

1- No

2- No

3- Sorry, but this is simple answer. No cost to use admob or mopub in your games, the advertiser can pay, I don't think there a risk for you.

thank you