They're on to us, that's why! The M50 used to go for as low as $110 on Amazon. Now you can't find it for much less than $160. Get enough people buying something and the deep discounts go away.
Or there could be some other factor that's causing the increase, since a lot of headphones seemed to go up in price recently. Small, on-ear portables like the PX100 II and PX200 II were selling well below their MSRP a few months ago. Recently I looked because I was searching out cheap headphones to recommend to a friend, and now they're all back up to near (ridiculous) MSRP. I also remember idly toying with the idea of picking up one of these for myself at the beginning of the year, but I need something like that like I need a hole in the head, so I didn't bother.
Oh, and yes, it's definitely worth checking out. Granted, so are all the headphones mentioned in this thread thus far.