I bought a 5 dollar kids party one from walmart( it is fabric not paper) and am going to fix it up Haunted Mansion style with cheap ribbon/flowers etc. I might have spent 10/15 bucks total.
Pinterest can give you all kinds of ideas and how to diy. Even before Pinterest, I figured that a decent headband, some felt or fabric, scissors and glue could make some decent Mickey ears, add a bow and you have Minnie.