Number of Advertisers
Another reason for your dismal Adsense earnings, even when the keyword has a legitimately high CPC, is that there are very few advertisers in the Google system paying for that keyword phrase, maybe as low as 5 or 10. Google will fill all 4 or 5 slots in your page's Adsense ad, but chances are that only a few of the very highest traffic sties with proven above-normal Click Through Rates (CTR) will get served those premiere advertisers. Your site is getting the tail end of the advertisers paying much less for more-or-less related keyword phrases that you probably never considered when planning your website.
A top-of-the-line Adsense keyword research tool will solve these problems by providing exactly the information you need to successfully build an Adsense site: (1) the Average Cost Per Click (ACPC) that all the advertisers in the Google system are actually paying for specific keyword phrases, not just the top bidding few, and (2) the number of advertisers actively bidding in the Google system for a particular keyword phrase.
The third element that a top-of-the-line keyword research tool will provide is the number of publishers competing for any particular keyword phrase. Well, you say, they all do that by reporting the number of results pages in Google. That is wrong and misleading. You may have avoided optimizing for some legitimately lucrative keywords because of this bad intel. Here is an example. Search the broad-match keyword phrase "best mortgages" on Google. You will get more than 65,000,000 million results pages returned. Try the exact match [best mortgages] and you still get more than 8,000,0000 results.
Try this eye-popping tip. Search Google with this search term (without the quotation marks) "inanchor:best mortgages allintitle:best mortgages" Now you can see the real number of competitors you would have if you decided to optimize for "best mortgages." Not 65,000,000 or 8,000,000 but exactly 637. The reasoning is that millions of web pages will contain the word best and the word mortgage somewhere on the page, but only a few will use "best mortgages" in the title and in the anchor text linking into that page. Those few are your real competitors, and not all of them will be running Adsense ads. You would never learn this critical information if you are doing keyword research that shows only results returned.