What kind of cosplay are you into? What do you like?
A fun and easy way to go is just pick the color palette of your favorite character, and maybe one distinguishing feature or prop, but just put together the costume out of regular clothes from the thrift store. For example, we had a "Grown Up Disney Party" a few months ago and I went with "Business Casual Maleficent." I made the iconic horns out of construction paper and wore some vaguely dramatic makeup, but for the costume I just did like a black blazer/pencil skirt with a purple blouse. My friend was Ariel as an old lady, so we just got a her a raggedy red wing, dressed her in old lady clothes in the Ariel purple/green palette, and threw some nautical themed pins on a blazer.You can do this with any genre, obviously cartoons or comics or video game characters lend themselves most easily to it because of the recognizable color schemes. Or, you know, "The Dude" is always an easy cosplay...