Well, for under $100, your only choice for wireless is Bluetooth v2, because any BT below that does not have enough bandwidth for decent audio. You'll have to take note of the possibility of lag time with BT. There's also RF headphones, but they essentially suck, due to interference and such.
If you absolutely must have wireless, look for an infrared solution (though avoid the DIR800C; amazing sound, but the phones snap easily). The good ones aren't cheap though, >$250.
Now, if you're willing to do some modding, maybe you could grab a cheap set of IR phones/receiver with 40mm drivers in them (saw some on eBay around $100). Pop out those drivers and replace them with the ones from Koss' KSC75, and you might actually have a decent set there for cheap. Though, this is just an experimental cheapo solution. No idea whether it'd work well or even at all.