The S400 and the Uptowns are both closed. If you're looking for open headphones for under $100, then you don't have too many choices. The Superluxes, SR60/80i, or lower end Senn's like the HD518 are commonly recommended. You'll have to pick the one that fits your sound preferences the most.
I'll throw in a rather obscure set of cans: The AKG K142HD.
Harman is selling off some refurbished models on eBay right now and you can easily snag one for under $50 (http://www.ebay.com/sch/harmanaudio/m.html?_nkw=&_armrs=1&_from=&_ipg=&_trksid=p3686).
Semi-open, relatively easy to drive, comfortable velour pads, super light yet rugged build. The sound is surprisingly good, and my favorite among the mid-tier AKG's (K240, K172, K272, etc). Has the most bass extension and impact of them all, to the point the midbass gets a bit wooly-sounding. They're still mid-oriented and the trebles are a bit laid back, just like the others. Soundstage isn't as wide as I'd expect from its "semi-open" design but it's very cohesive with a good sense of depth.
I can nitpick about the sound quite a bit but at the $50 price point they are quite the bargain. I also think it looks very good- the matte steel over shades of green is a pretty pleasing combination.
Just my 2 cents.
Edited by viralcow - 3/11/13 at 3:15pm