I agree on the ATH-M50 headphones, they are very nice phones and probably could be your best bet for your budget. I just got a pair recently and like them a lot. I've also got a pair of Grado SR-225 phones which I think are nicer cans and sound better to my ear, but I don't really like the Grados for mastering. The AT's would be better suited to mastering decisions in my mind.
That said, I've never personally felt comfortable mastering or making mastering decisions based on headphone listening -- just doesn't work for me. So while the ATH-M50's might be your best bet for your budget, if you want to think about a speaker-based playback system instead, here's a option.
I'm a big fan of T-Amps (Class T amps, based on the tripath chips) -- they sound very good period, and for the money they are killer. Do a search on them, there are a few threads on them here in the playback section. I've actually got 3 T-Amps in various systems, all from Hong Kong/China, 2 from ebay. Pair the T-Amp with a pair of bookshelf speakers. Used obviously would be a better bet given your budget. I've got one system using a T-Amp and a pair of Polk Audio R15 speakers (also found as RT15), which are nice bookshelf's for little money.
I also find it useful to have a subwoofer in the playback system, since bookshelf speakers don't put out a lot of bass, and the shows we record probably have lots of bass.
That all said, here is a budget system I'd put forth, all new and easily purchased. A bit over the high end of your budget, at $185 shipped for everything. I'd bet it would at the least work ok for what you are doing, and may even sound pretty good considering its very low budget basis:
- T-Amp from ebay, this one should do nicely ($30):
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=320524272040- MTX bookshelf speakers/monitors from parts-express ($60):
http://www.parts-express.com/pe/showdetl.cfm?Partnumber=302-019- Dayton sub from parts-express ($95):
http://www.parts-express.com/pe/pshowdetl.cfm?&Partnumber=300-631Free shipping on the parts-express stuff (over $100 free), and the Dayton sub has both RCA inputs and outputs, useful with the bare-bones t-amp. Computer soundcard out > RCA inputs of the Dayton sub, Dayton RCA outs to T-amp RCA inputs, MTX speakers connected to the T-amp. Other than a couple cables and some speaker wire that you might have lying around, $185 for the complete system including a subwoofer.