The headphones with the best overall sound quality to me under $50 are the Maxell DHP-II. Unfortunately the build quality is poor, but i've heard they now have the DHP-III out. So far mine has held up well. For being only $30 I wouldn't mind if they broke after a year. I'd just buy another pair. I don't like to suggest them due to it being a risk. The sound quality is so good that's it's worth it.
The only thing that almost comes close is the Beyer Dynamic DT-235 for $45 or so. They're a bit goofy looking but they sound pretty good, but they do lack bass somewhat.
To me, both of these have better sound quality than the UR-55. UR-55 definitely has the most bass out of all of these though.
I actually think the DHP-II actually sounds a lot better then a lot of $100 headphones. Listen to them and it's hard to believe it's only a $30 headphone..
One headphone that's dirt cheap that has excellent build quality is the ATH-M30. I got mine for $55 and you can even use velour pads with them! They don't sound anything like the M50 though and they're fairly neutral with more forward mids compared to the M50. Some may find them quite boring and they do need an amp to sound best. I found them very comfortable.
