The options are scary. There are quite a few threads with "which headphones for $$ this much $$?" over at headfi.
Start here;
http://www.head-fi.org/forums/forumdisplay.php?f=2
I would say that there are many good options out there for under $100... and some real bargains in the $20-$60 range.
Next level of cans (headphones) would be those in the $80-$150'ish range.
You can also get some great deals on used kit over in the headfi forums... sometimes 50-60% of retail.