Craigslist is the student's best friend. With your 400 bucks don't forget that there are many factors that dictate how comfortable you are with riding your bike. You will want a bicycle commuting specific bag, a big blinky taillight, a blinding bicycle headlight, a helmet you like, U-lock, mirror, and I like to commute with cycling specific clothing so that I can change into a sweat free t-shirt and jeans