There are 4 major factors determining what you earn in AdSense.
As mentioned above, niche is a determination factor, as advertisers pay more on some keywords than others. So this is a big determining factor to your earning. You may want to use Keyword Planner offer by Google AdWords to find out what you may likely to gain from you keywords on your site. Note that you are given a percentage of such amount offer by advertisers, depending on what account type you have with AdSense.
Secondly, the country of your visitors is also a great determination factor to what you earn. In some countries, the highest you can get per click is not more than $0.09 while in others, you can get up $5 per click.
Thirdly, the quality of content you have on your site. No advertiser will want to bid high on a site that doesn't add more value to the visitors, though the bidding is done automatically by Google AdSense, a lot of factors are used in evaluating quality of content.
And lastly, the number of click through you get daily. Because even if Google AdSense pay $100 per click and no one click on your ads, you might not get a dime. So knowing how and where to place your ads are also determining factor for what you earn.
So in a nutshell, only by practical, you can know how much you can earn.
I hope it helps.
