Around $150-300/mo. with http://testyourvocab.com from Google AdSense. Tells you the size of your English vocabulary. And it was HN which popularized it in the first place!
I really like testyourvocab.com. I use it from time to time to check my progress.
Cool site. I wonder if you would make more on AdSense if your advertisement was above the fold.
just checked out your site, i think its decent it would be great if you made it easier to tick the words, its difficult to pin point and click too many check boxes