Are you at Uni?
Doctor is easy - borrow a labcoat, chinos + light blue shirt and then hit a charity shop for cheap Doctors & Nurses accessories, or even Woolies have them for a fiver or something.
Dracula is another good cheap one, wear a tux with dress shirt, then white face-paint, slicked back hair and fangs, with a bin-liner cape, easy
Detective - do you have a long beige coat? Charity shop a beige hat, charity shop dark shades and walk around with a notepad and pen
Umm that's off the top of my head, will think
*edit*
OK Dolly Parton - blonde wig, dress, massive jugs
*edit2*
Haha, David Dickinson - Suit up, chazza shop glasses, orange face paint, job done
*edit3*
If you've got a mate, go as Dumb & Dumber, that'd be pretty cool, haircuts and all!