I'm not the most knowledgeable member, but I do know basic things to know before recommendations are what your source is, what type of music you listen too, and if there are any particular sonic qualities you are looking for(big bass? no sibilance? big soundstage?). How essential is comfort? Should they be portable, and can they be driven without an amp?
There are many generic suggestions. My personal you-can't-go-wrong-with-them cans are the JVC RX700s, especially after they are burned in and lightly modded(don't even need anything except your hands). 35 Dollars dollars so your wallet isn't obliterated. Many think they are the best cans under 100, or at least in the upper echelon. They have a slightly warm sound, and no harshness but plenty of detail. Bass tight and quick while still having some physical impact. Mids shine on these in my opinion after the felt mod, and they sound veryyy smooth(yumm violins).
Some other overall closed suggestions:
Sennheiser HD448s: have a more neutral sound, but the bass isn't as articulate.
The Denon D1001/Creative Aurvana Live: Essentially the same headphones(share the same drivers and pads), these have big bass. Some say they have recessed mids when not powered by the right amp, and harsh highs, but still a great value, especially for the CAL.
K81DJ: Big Bass, without losing too much of the others frequencies, especially after mods. Apparently are initially very tight, and can be quite uncomfortable at first. Provide great isolation, and are very compact.
PX200-II: Foldable into a very small(and pocketable size). Convenient volume control on the cords and great sound for such a small package. Significantly better if amped. Very comfortable and feel very sturdy. The sound is like a more neutral RX700 with less palpable bass and a bit less detail/speed(although this seems to equal or almost equal the RX700s with amping). I love these. They sound very airy for such small closed cans.
FC700: Very underrated cans in this price range in my opinon. Bass is nice and big though somewhat slow, perhaps bloated, but not bad at all. Are very sibilant at first, and still retain some of that harshness even after burn-in, but it barely bothers me anymore. Vocals shine. Impressive soundstage for being closed. However you might want to go for the upper class ES7s instead since your limit is 100. Haven't heard those though.
Check out the Shure SRH440 and ATH-ES7 out.
I don't know much about open cans, but the Grado SR60 or SR80 are always top recommendations for open cans, especially if you're big on rock type music. The HD238s are supposed to be quite good as well. THe yuin G2As are clip on that are supposed to sound awesome.
More details about what you are looking for would certainly help!
