To make money with Ads, you need a LOT of downloads and a lot of impressions.
On average, a banner Ad with Google AdMob could earn you ~$2 (varies) for every 1,000 impressions assuming you get 2% clicks. If no clicks, no $.
You need tens of thousands of impressions per day for it to start generating some ok income, and hundreds of thousands of you want decent income.
Something I've noticed, the click rate % is highly dependent on the audience type. Casuals will click, hardcore gamers will not.