Also very important is that the cost to play for people landing on the ad is zero. Somebody should be able to see the ad, click on it, download the client and play, and be able to make something without having spent any money.
The ads would be cheap but the hunters for zero cost playing expensive. It would be sustainable if coins for free hunters come from the game itself. And/or if there are very few free hunters per day and clicking one of the ads is like a free lottery, not a guaranteed hunter.