Hahaha, it's usually the other way around...
The best sound quality under $100 is arguably the Grado SR80. But this will not insulate you from outside sounds at all. So any other headphone is going to drop one of the criteria you want, quality. How much? A fair amount.
The V6 is a bit of an aberration which is one of the reason why everyone recommends it. It is basically a $100+ phone being sold at much less. The quality of the sound still doesn't come up to the SR80 but it is closed, reasonably portable (folds), reasonably easy to drive and reasonably comfortable. The V6 has a good bass, flattish mids, a raised high-mid (which makes the phones sound a little strident and also makes people say they have a recessed mid) and occasionally shrill treble. It is not the most relaxing phone to listen to but it does let you know what is happening with the music quite faithfully.
Comfort is one of the most subjective things on this board, and one person's nirvana can be another's vice (literally). I find the V6 (also known as the 7506) only marginally comfortable.
The PX200 is a good phone for the money. But it is not a bargain... You pretty much get what you pay for. A reasonable sound for that class (although by no means the best) but with the main emphasis being on portability. If your head fits it (mine was too big) it is very comfortable to wear for long periods. The design is also very slick. Unfortunately it does not seal out much in the way of outside noise.
The Sony MDR-D66SL "Eggo". Around $100. Not such great value this time around, mainly because you are paying full retail and shipping from Japan. Once again like the PX200 it doesn't block out a lot of outside noise, although in this case it is a deliberate part of the design. The sound is evenly balanced and generally nice... Nicer than the V6. Not for the bassheads, more for the omnivore. It folds into an eggy shape and is quite comfortable to wear for a while as long as you tweak it a bit.
Sony MDR-EX70/71 in-ear phones. I had abandoned these a while ago, and I was surprised when I wore them again recently how relatively little they isolated in comparison to many closed headphones I have/have had. It's not that they don't (perhaps it's a little, but only just a little better than the V6) but seeing as you are putting earplugs on I expected more. Sound is muddy and very bass heavy. For me rather unpleasant to listen to.
Shure E2c. I've ordered the E5c and am eagerly waiting to see what they're like. Unfortunately I have no idea what the E2c's are like. These don't seem to isolate a huge amount (only about the same as the EX70 given the specs) but it should be useful isolation.
Also, good semi portable closed phones over $100 which will work out of the laptop and Slimx:
Ultrasone HFI-650 (omnivore), Sennheiser HD25-1 (bass emphasis)
As it happens I have an HD25-1 for sale at $130 shipped...
) However, quality is really the most important thing I'm after.
