I don't think you canf ind the AD300 for under $50 can you? I got mine on sale for $36 at Wal-Mart about 6 months ago and for even $60 it's a STEAL!
It's way more comfortable than the AD700 because it's smaller and feels more secure. Still has those scratchy pads.
I prefer the sound to the AD700. Mids are more forward and sound fuller. A little less treble with more bass. Sound is also a little less detailed and less clear.
Would be the perfect budget headphone. I personally think it's worth $75 if I had to use only one open headphone under $100.