You can use a combination of
asRep and your Adsense reports to determine exactly which clicked ads pay how much. As for it being against the Google Terms to identify and filter out low paying ads, this is news to me. I've read where Google
suggests that you don't filter out low paying ads but I've never actually seen it explicitly forbidden. Somebody correct me if I'm wrong please, i.e., cite the relevant part of the Google Terms and Conditions.
I used to filter out low-paying ads but soon discovered it was kind of like shooing away flies. They seemed go away for a little while but sooner or later more would just come buzzing around. I soon discovered that if I maintained that agenda, I'd use up all of my allotted URLs (200 I believe) in my competitive ad filter list in just a few months. Also, some of your low-paying ads could also be your best performing ads and if you filter them out then you could be shooting yourself in the foot.
Now I just use the competitive ad filter to get rid of all the scraper sites and crappy made-for-adsense sites. The asRep script (see link above) is pretty good for zeroing in on those as well.