Here are some headphones I have owned that cost $150 or less that I like more than the M50x.
open: STAX SR-30, Philips SHP9500, Sennheiser HD 590 Prestige, Audio-Technica ATH-AD700
semi-open: Koss KTXPRO1
closed: Yamaha HPH-MT220, Audio-Technica ATH-M40x, Audio-Technica ATH-M30x, Sony MDR-V6, Sony MDR-7506
There are many more, but these are the most notable ones. Note that some may no longer be available for under $150 or even at all. Generally, to get the best sound for your money, it's a good idea to look around the used market, as this will let you reach into higher price points at the same budget.
A few comments:
The MT220 blows away other closed headphones I have heard and even many open ones.
The KTXPRO1 is sometimes the most exciting headphone I've heard, yet it only costs ~$15!
The SHP9500 benefits from
this modification. The AD700 sounds similar, but not quite as good to me.
The SR-30 is uncomfortable and not the most neutral headphone out there, but has a ridiculously transparent sound.
The M40x and M30x are less bassy than the M50x and sound better to me...but if I were to equalize all of them, my preference may change.
Both Sony headphones are far too bright and harsh for general listening, so if you go with either of those, I would advise generous equalization.