Saying it's nonsense is a somewhat bold statement. Even if it's subjective most would say that Beats don't have great performance at their cost. And yes it is somewhat diffucult to put it in numbers how much better something is. Overall I prefer HD-650 over the HD-598 but both are performing well. HD-598 even performs better in some instances so all in all it's not far off.
You also have to consider that headphones can reach better performance when connected to better amps and dacs. It's difficult to compare portable headphones to full size ones. HD-700/800 has a hard time reaching it's full potention out of an iPod Touch. In this case maybe even the PX100 would sound better.
Some would say that the price range 300-500 USD is where many hifi headphones are at. Above that is where things like distortion values, vibration dampening, quality material play a greater role. Usually there is a lot of research going on perfecting something that is already considered good and all of it has to pay off in the end hence the higher price tag (that is what I think).