I am about to start an ad campaign for my site of gaming niche. Is there any average ROI that I can expect from it? I am going for Adwords for my ad campaign.
I am sorry to say that no can tell you the average ROI for any niche because there is none. You have to calculate it for yourself which one seems good to you. First invest a small amount of money, see how much conversions are you getting from it, calculate the profit and then decide whether or not you want to continue or rather change your ad campaign.
Nobody can tell you that because there are none. You have to calculate it by yourself. Begin with some tests if you make a profit then considered as a general ROI. And then start experimenting with your campaigns.