One year I forgot all about Halloween because of other issues and at a last minute- literally- I grabbed some purple balloons left over from a bday party and safety pinned them to my sons clothes. He went as a bunch of grapes, although I called him the Fruit of the Loom guy! I also took a brown paper bag and made that into the vine and put that on top of a hat to finish the look. Was adorable for about 10 mins tops of work!
Another year I made him into a box of Popcorn. All I needed was some CHEAP poster paint from the dollar store, a box and some pocorn and glue. On the cap I glued tons of popcorn to it and the box I cut out a hole for his head and arms and glued popcorn to the top of it and put red and white stripes on the sides like a movie popcorn box. Was a bit time consuming but he won first place for this costume in 2 contests!
Maybe you can find a way to also make a soda, butter and salt or candy costumes quick and easy! Make it into a night at the movies theme!
I can not wait to see other ideas here since I am looking for a affordable one this year for my daughter!
Good luck and many blessings!