I like putting together costumes from thrift stores. They aren't cheap material like those horrible Spirit Halloween costumes, and they are still more affordable!
Just some ideas of what my husband and I have done in the past:
The Joker (Yes, we found a purple suit jacket, orange pants, green shirt, and colorful tie)
Gypsy (This is super easy, just lots of scarves and pieces of fabric hanging)
Steampunk (Lots of details, but I've been building it up over the years)
Viking (We found a toy helmet)
Evil Queen Dress ( I made a giant black tulle skirt from tulle I bought discounted and a fancy black top) Also, ebay is your friend if you do it early enough with China shipping
Harry Potter Wizard (I dressed as Bellatrix and made my own wand by sanding down a stick w/ hotglue & painting)
The killer from Scream (You always find that costume donated everywhere)
To sum it up, just go to a thrift store and see what's there, or have an idea and go search for pieces to put together.