Sorry if this has already been posted, but oldnavy.com has many of their halloween costumes marked down to $17.20 with an additional 30% discount making them $12.04 each. Free shipping with an order over $50. 30% off ends on 10/2/12.
In my experience they go a little lower than this right before halloween, but in my opinion this is a great deal. I think their costumes are very good value for the money.