Carmina, some of the Simplicity/McCalls costume patterns do not call for any sewing, just lots of hot glue--ha!  Seriously, some of them are pretty simple, using felt or fleece pieces.  Just make sure you get the pattern during a 99-cent sale at your local fabric store!

Also, my best friend and I traded costumes back and forth several years.  She owned a mouse costume that all of our kids wore at one time or another!  I know I have made costumes for other people's kids--maybe you could barter for them.  I've had people offer me fabric for my use if I would construct something for them and I've done that many times!  Miss Paula in Texas

Wow, Thank so much everyone for the great Harry Potter ideas! Like some of you when I was little my family never had the time or money to get me a Halloween costume. I can't tell you how many times I threw a sheet over my head and went as a ghost or threw on some worn out clothes of my grandfathers rolled up the sleaves and legs, tied a bandana to long stick, put some grime on my face  and went as a  hobo. I don't think that would be politically correct any more and you would have to use the term vagrant or homeless person instead of hobo LOL  So I guess I am living my childhood over again through DS this time with really cool costumes.

Last year they had a costume contest at work. I cut a hole in the bottom of a rectangular laundry basket, stood in the hole, pulled it up to my waist, filled the laundry basket with clean laudry, a box of dryer sheet, stain remover, and a bottel of laundry detergent, I clothes pinned a bunch of dryer sheets to my shirt and went as a basket of clean laundry. I won first place 25.00. I smelled good too LOL.

Thank you for the information.  I will looki into the patterns, no sewing... hurrah!

I just got this tip off my local freecycle board for a baby or small toddler:

Find a stuffed animal a little bigger than your child. Cut it up the back, and remove the stuffing. Cut out the face, leaving the ears and back, and stitch or hot glue your raw edges. This makes the hood. Add Velcro where you cut the slit. Voilà! Almost instant costume!

I thought it sounded fun and easy. A great way to reuse stuffed animals (that we all have sooooo many of).
 

I have sewn a dinosaur costume that has been worn by 2 children (next year yds will wear it).  I knitted yds an orange sweater, pumpkin hat and a pair of striped tan and green pants.  He loves hats and wears them constantly, a sweater can be worn all winter and the pants double as his nighttime cloth diaper cover.

1 year I took a purple teshirt and drew black triangles all over it.  I added black pants and put a chicken bone in my hair.  I was Pebbles from the Flinstones.

1 year I was a 2 headed person.  My mom found a maniquin head and glued it to a box, we cut places for my arms and head threw a shirt over it and pulled my hair and the maniquins into a pony tail.

Last year ds wore black and had fake money and fake jewelry hanging out of his pockets.  He also carried a canvas bag with a $$$ sign on it as his candy bag.

The year before ds was a dead baseball player.  He wore his uniform glued a baseball to the top of his hat and added fake blood.