I think witch is the easiest, I've gone that a couple years. Black shirt, Black skirt, black nylons, Big black clunky shoes.
I can find all that in my closet. Buy a $5 witches hat. Add some makeup and there you go. Same thing works pretty much for a vampress. Add bright red lipstick, black eyeliner and fake canines.
A farmer. If you have overalls in your closet. Get a straw hat. Put those handheld shovels and rake things you use for gardening in the back pocket as well as some long grass. I thin rubber boots would complete the outfit, but shoes are fine.
A little kid is easy. Mascara to make your eyes look big and freckles.
Almost every year I look in my closet first and create something and then buy maybe twenty dollars or less of acessories.
Hope it helps!