HGTV online has some good ideas and free stuff to watch right now, too.
This is one has $100 ideas under $100: http://www.hgtv.com/hgtv/pac_ctnt_94..._31052,00.html
Let's say you can muster up $200 for your bedroom:
I'd say paint the room, you could probably do that for $40.
Buy a new comforter or duvet cover, hopefully you can scour some sales and pay $50 for one. (It would be even better if you have something in your house already). I have seen brand new comforter sets at garage sales, in the packaging, that regularly cost $300 new (I knew that because I had just bought one, the same brand, for that much), so you never know, you could be REALLY lucky. Again scour some sales and find some throw cushions for the bed, or reupholster some that you already have. If you have a chair or piece of furniture that you could throw a slip cover over or could reupholster yourself with a screwdriver and a staple gun, cool. I just recovered 6 chairs for less than $15 and I have fabric left over.
Scour garage sales, Sally Ann and Goodwill, freecycle ads, newspaper classifieds, etc., and find any other furniture you might need, perhaps dressers or a sitting chair, something like that. I know that I recently saw a free cherry bedroom set given away on something like freecycle. We have also helped a friend "trash pick" two different solid wood dressers, both very nice...one didn't need anything and one became gorgeous with a coat of paint. It's pretty cheap to add attractive draw pulls as well, if you want to spruce something up. Add a couple of inexpensive house plants and use some paint to paint your own art for the room, whatever you can muster, even if it is abstract or geometric.