In 2009, I had a post-apocalyptic graveyard theme for a Halloween party. It was very cheap to put together and very effective. You don't have to spend a lot for a Halloween party, as the idea is to make it as dark and rugged as possible which means repurposing junk.
The first project for my postapocalyptic graveyard was the toxic swimming pool. I took gallon water and milk jugs and put toxic symbol labels on them.
I then poured some green and yellow fluorescent paint inside and rolled them around until coated. I bent a green glow stick too for extra light and tossed it inside. I sealed the lids on and tossed them on the pool surface.
I put a green lens cap over the pool light. The walls around the pool were decorated with spider webs and I took 6" round styrofoam balls from the craft store and painted them black and used black pipe cleaners for legs, making giant spiders to crawl around the webbing on the walls. Toxic dump complete.
Guests entered a side gate, forced to come into a side yard. I took apart an old white garden lattice and made over a dozen white crosses for that yard, painting them with green fluorescent paint (99 cents at craft store) and setting up a black light (a long black light tube from Spirit Halloween about 29.99) so they glowed. I made a mannequin with a gas mask my son had in his collection of weird stuff and radiation warning symbol on his jacket to "greet" them as they entered the toxic pool area.
The patio was set up with a spread of great foods and a wheelbarrow filled with ice and beers and wines.
Using this theme, I was able to cobble things together that I already had without spending money on decoration, other than fluorescent paints and a black light. The patio area near the pool, I took old drop cloths and tattered the edges and stapled on like curtains with moth holes and frayed edges.
In the main yard, I made a fire pit and seating area, a fake cemetery in the corner, but also took baby dolls and put them around the shed with a sheet hanging on it. I took a projector and projected a creepy stop motion animation video by Brothers Quay ("Street of Crocodiles") and it created a disturbing look as the dolls I set up interacted with the ones in the film.
The bathroom was decorated with newspaper cut outs of murders and bodies found and pictures that looked like yearbook photos of potential victims. It was put like a crazy patchwork on the wall with some bloody fingerprints and evidence of a bloody body in plastic wrapped up in the bathtub.
The actual decorating for the party cost almost zero for me, just a few dollars. If you pick a theme like this, you can make it cobbled by old stuff and it's even creepier. One section of baby dolls you can buy in groups in bags at the Goodwill, can make a nice little Doll Island setup.
Pieces of old cars, junkyard stuff, can be really creepy and sketchy, like the movie "Wrong Turn."
A hillbilly theme with a keg and mason jars, a moonshine bar (put your liquors in glass jars with labels) and a lot of "junk" in the yard and a sofa moved outside can say it all. Add some bayou netting and the sounds of crickets and tree frogs, a little banjo music and your deep in the swampland.
You can also build the entire party around the firepit with firepit prep foods, s'mores, perhaps some creepy moody background music and urban legend and ghost story telling.
Toss a lot of white sheets from a thrift store over the furnishings, set up some battery-operated candles, lightning flashes in the corners of the room, thunder sounds, and have a nice haunted house theme.
Tea-staining sheets, roughly sewing together a patchwork of worn out fabrics to make creepy curtains, duct tape, cardboard, styrofoam with plaster of paris to make headstones, old clothes to stuff for dummies....
Let your mind create an atmosphere that is creepy, uncomfortable, and weird. It doesn't cost a lot, might not even cost anything if you can pick through your family and friend's shed contents, but you can have an excellent Halloween party and save your money for the booze and food.
You can find endless ideas with details in my book Adult Halloween: Taking Back the Season!