Cheap, easy and portable:
Make yourself some scarecrows. You can make them look like various monsters. Use lightweight PVC pipe for the frameworks. Goodwill clothes to make the costumes. Benchpress type weight discs to hold them upright.
Styrofoam tombstones are cheap, light and easy. If you get the high density stuff it can be carved with a router to make it as elaborate as you want.
Hanging bats. PVC frame work, black trashbags and/or cheap black cloth. Use styrofoam balls for the heads. Hang them with fishing line.
If one of you is artistically inclined, make some movie poster replicas out of posterboard and the cheap poster frames you can get from Wallyworld.
Hit up a few party supply stores, they usually carry a limited selection of halloween stuff year round, and you can probably pick up a few cheap table decorations there.
Hope this gives you some ideas.
Gale