I would greatly suggest the K240 over the M50. I have both and the K240 is much better.M-50's sound good out of the iPhone, been there done that. If you're lucky, you can get them <$100.
Perhaps you could also consider Sennheiser HD 239's. If you don't like how they sound, they can be easily and safely modded (search HD 238 modding). BTW, 239's are only a cosmetic update over 238's, so you can perhaps get cheaper 238's.