There is a really good deal on a Trek 470. $230 for a brifter bike. Will not last long. Yes, I know that is more money, but at the $100 price point, you are going to find junkers, unless you look really aggressively (look past C/L, pounce on deals, etc).
Yeah I technically have a budget of zero cause I already have an around town bike I'm working on, but I figured if I saw a really good deal I might be inclined to pick it up. There's also a Raleigh fixed with a flip flop hub for 150 obo. http://nashville.craigslist.org/bik/2317082165.html