Well, your choice in bikes is limited by your budget, which is not a bad thing. Not many local bike shops will have a bike under that price, unless they sell used. Your main choices are going to be comfort, cruiser, road and mountain bikes. Check craigslist.org for local used bikes, you can stretch your dollar with a used bike. If used is not appealing, head to a retail store of your choosing.
For example, Walmart sells a 26'' Huffy cruiser for $99.97. It's a single speed bike, with a large comfy seat, fenders, and a frame that is easy to step over.
http://www.walmart.com/catalog/product.do?product_id=5983270
I think a bike like this is right for you. The large tires will handle the urban streets well, and the single speed is worry free. Make sure you check local laws and ordinances regarding bikes, and get a light for the front and back. Registering your bike with the police dept. is also a good idea.