Unfortunatly for you, December is the most expensive month to stay in Brussels (Christmas shopping). You have to book as soon as possible.
Never tried, but look at this website. Their infos are quiet clear (promotions, pictures, correct location map) and seem to be medium : http://www.holidaycityeurope.com/brussels30/index.html
Of coursen, the cheapest way to stay in Brussels is a room in a youth hostel. Here : http://www.laj.be/
You can book a single room (en-suite bathroom) for 30 euros / per night, like (click on Brussels, then Brussels J. Brel or Brussels Europe).