Try taking advantage of all the free things a University has.
- If you have free printing at your college, - then go wild with printing off cute pumpkins and bats to hang around you room. If not, there's still usually some scrap paper to be found with something printed on one side and a canvas for you to draw Halloween masterpieces on the other.
- News papers are abundant on my campus...maybe there's something creative to be done with them?
- Look for a listing of events offered by your school, I know my dorm last year had a gingerbread house contest, and they supplied all the supplies for you.
- This might be kinda gross...but my dorm had huge recycle bins, and there is probably something creative you could do with a bunch of empty plastic bottles (see: http://www.craftster.org/forum/index.php?topic=274562.0
- free white t-shirts could become spooky ghosts
Just look around you when you go to class these next couple of days and see what you can find
Some other cheap ideas:
- a dollar bag of halloween balloons or crepe paper could add a bunch of festive color
- decorating the outside of your door (it's a small place so cheaper than your whole room, and you'll see it every time you come back from class)
- go wild with a cheap pack of construction paper (last year i bought one and used it for all the major holidays)
- utilize craft supplies you might already have, - scrapbook paper, paint, etc.
Sorry I couldn't come up with anything great, but maybe something will give you inspiration.