Ok here's the requirements:

Budget: $100
Room Size: Normal size bedroom but I cannot put the bass too loud as I still live in my parents because I'm 18 in college, so they would be annoyed and maybe neighbors too.
Size requirements: 8 or 10" but probably 8" as 10" are usually above $100.
Primary Use: Gaming with Xbox 360/PS3/Wii and movies, some tv.
Appearance: Doesn't really matter as it will be on the floor in the corner where no one can really see it.

Is it possible to get a nice subwoofer to add to my surround sound system to make it 5.1 for under $100? Will I really notice a big difference from my 5.0 bedroom system? I've been looking on Amazon and found a Yamaha and Sony 8" sub for around $75-100.

My setup right now is a Sony surround sound system 5.0 that I paid around $350 total for 5 nice big speakers and 6.1 receiver.

I had the little Sony SA-W2500 (http://www.sonystyle.com/webapp/wcs/stores/servlet/ProductDisplay?catalogId=10551&storeId=10151&productId=8198552921665089844&langId=-1) while I waited on a made-to-order sub. It was good for what it was, and would be very nice in a small bedroom as you describe. It also meets the price point. I bought it at CC as an open-box for $80. You might want to look at e-bay, pawn shops, open box deals, etc. too for cheaper subs, probably even better than the SA-W2500.