I really like '80s and '90s rigid mountain bikes for commuting. You can set them up a million ways, use fat tires, and they're super durable. In Minneapolis on any given day you can find an old mountain bike that could be fixed up and ready to ride for a total expenditure of $100-$150. Here are the two Schwinn mountain bikes I've ridden recently.
IMG_4469 by [url=https://www.flickr.com/photos/53223642@N05/]


bike by [url=https://www.flickr.com/photos/53223642@N05/]


Each was under $100, and they make the best winter bikes!