[oblique63]

I just posted about these in another recommendation thread earlier today, but they seem good enough to bring up again; the Denon HP700's.  I haven't listened to them personally yet, but from the descriptions in this thread and this comparison, they seem like a solid choice, especially if you want more clarity/detail then the m50's. They're only $99, so you still have money to spare, and they're supposedly comparable to much more expensive headphones. Unfortunately they seem to be a bit unknown/underrated, so there aren't gonna be a lot of first-hand-experience comments outside of those two threads I mentioned.
 
Besides that though, I'm not terribly familiar with what else is available at that price point. Just make sure that you end up with something that has good 'mids', cause that's the most important frequency range if you're going to be listening mostly to rock.