This is pretty easy for most women because we all should have a black dress in our closet, right? I paired mine with a vintage cape (I mostly wanted an excuse to wear it), tights, black boots and a witch hat that I got for only $3 at Target. Total cost of the this costume: $3
I own about 7 plaid shirts so this costume was a no brainer for me. If you don't have one, they are relatively cheap at the thrift store. I also added my brown lace up boots, black beanie and my buffalo plaid jacket. This is a perfect costume for Halloween in the Midwest because you are already wearing lots of layers!
Wear a pair of khaki pants or a plaid skirt. Pair it with a blazer, oxford shoes, glasses and a bun. Think Indiana Jones or a stuffy British professor :)
Mad Men Character
You can pick whichever lady (or gent) suits your style or what you already own. Since a lot of modern style is taking its cues from the 50's and 60's right now, you might have something that would work even if you don't own any vintage clothing. I would wear a 50's dress with a full skirt and go as Betty, a pencil skirt and tie blouse could evoke Joan or anything plaid or conservative could be Peggy.
Most importantly, have fun and get creative. My favorite costumes have always been the ones I've put together myself instead of buying. I would love to see what you come up with. Have a great Halloween!