Hugs to you.
There are a lot of websites out there with ideas for low or no cost costumes... but the cost often depends on what you have around the house or can find free locally.
One option is freecycle if you have a local group. Even if you can't find a costume on freecycle you may be able to get the "pieces" to put together a creative costume.
Some things I've done:
litter bug... make "wings" out of recycled cardboard (we used a pizza box lid), a "jumper" out of a trash bag (cut arm and leg holes, head came out the top) and plumped it out with crumpled paper. Made a headband out of aluminum foil and foil "antennas". My SIL wore this over a sweat suit, carried a cloth candy bag, and handed out recycling info bookmarks as she collected candy.
DD1 is going as a cat this year... black sweat suit, ears cut out of cardboard and held in place with clips, nose/whiskers drawn on with eye liner, and a tail made from an old black stocking with the end painted white (we have some white house paint in the shed for that "detail"). Any animal costume is about that easy... a top and bottom in the general color, cardboard or felt or crepe paper ears/tail/trunk, drawn on facial details.
Dollar stores and craft stores often sell crepe paper in big long rolls for a dollar or less. White crepe paper wrapped around his body and a little red whatever (paint, jelly, ketchup, fake blood) could make a scary/gross mummy.
Or dress in green/brown, make some "oobleck" (mostly corn starch if I recall) and smear it on his clothing/face/hair and use a big white pillow case for a candy bag... he could be a booger/mucus (which is nicely gross
A thrift store might have old prom dresses you could cut down for a princess dress (or freecycle a request for fancy dresses/bridesmaid dress/communion dress). Or more of that crepe paper could be stuck on a piece of duct tape (looks kind of like a "hula grass skirt") to make a full, flowing, funky skirt. Add a tiara (twist the crepe paper into a crown shape, or use aluminum foil) and scepter.
And two "baby" costumes I love are dressing the babe in black sweats and adding extra legs using knee high panty hose (25 cents a pair at walmart) to make a "spider". Mama or the care giver then dresses in black as well but paints spiderwebbing on their outfit (or use that fake spider web stuff) so that the baby spider has a web. The other was a baby in a green onsie/jumper with three big green "balls" attached to the front of the jumper (I think she used green tulle but anything green would work) to make a "pea in the pod"... Mama dressed as a plant (green outfit, big cardboard leaves and pipe cleaner "vines") and the Dada was a scarecrow.
Anyway, have fun! Growing up we always made costumes using whatever we had and we come up with some really neat stuff!