check out this site:

http://www.holidaycafe.info

they have lots of offers although mostly holiday based, but since its the owners themselves who advertise their places, i'm pretty sure you can arrange something...

 

Good luck

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).