A few do-it-yourself decorations that don't cost much:
GHOST: Inflate a round balloon or beachball. Put an old white sheet or tablecloth over it. Tie a piece of string below the balloon or ball. Cut eyes out of black felt or construction paper and stick on (you could also draw them with a black marker if you really don't care about the sheet). Hang from your porch or a tree in your yard. You can also make tiny ghosts to decorate a tree with using tennis balls and handkerchiefs or rags cut to the size you need.
TOMBSTONES: Cut heavy cardboard in appropriate tombstone shapes. Paint gray. Write scary or amusing epitaphs on them with black markers.
SPIDER WEBS: Drape white or gray yarn among your trees or shrubs or on your front porch. Add some...
SPIDERS: Paint styrofoam balls black. Stick in black pipecleaner legs.
SCARECROW: Stuff a pair of old pants and an old shirt with balled up newspaper. Have straw sticking out of the cuffs of pants and shirt. Add a pumpkin (real or plastic) for a head.