The point of diminishing returns is there, but it is somewhat subjective. It's subject to one's hearing and disposable income. That said, in my opinion HD600/HD650 are right there in the sweetest value for money price point. Cheaper headphones are not going to sound nowhere near as good and while better headphones might be slightly better, they will also cost at least twice as much.
What I want to point out though, if you're spending $300 on a set of headphones you should probably research to see which set of headphones match your sound preference the best. At the $300 range most of the headphones from reputable manufacturers (i.e. Sennheiser, AKG, Beyerdynamics, AudioTechnica, Grado, and even Sony) are going to sound really really good. The biggest difference will be in the sound signature. For example Grado is known for lively dynamic sound, Sony is known for putting emphasis on treble, Sennheiser's HD600/650 has a reputation of a very neutral sound. Beyerdynamics sound signature varies a lot with the model, the "DT770 Pro" model is known to be very bassy, however for my 2005 DT770 Consumer edition I had to use equalizer to tone down treble which in my opinion was too overpowering. So do your research guys.