Beyerdynamic 990'swould do the trick and should be in budget, they're open back so they sound pretty natural (the 770's are closed back and 880's are a semi open) and they're insanely comfortable to wear(feels like you're getting a head massage!) Although as a couple of people have said, its not really recommended to master on headphones, but if you are going to, then these puppies will do a good job!
I highly recommend the M50s. I use them myself for recording and for causal music listening. Fantastic headphones with a very natural response.
However, I wouldn't recommend using headphones for mastering, that should be done on high quality speakers. Mastering on headphones wont give you an accurate representation of the sound as they give you a very isolated acoustic environment.