Target offers the Star Wars Men's Jedi Knight Robe Halloween Costume for $51.20. In-cart, it drops to $39. Apply coupon code "FORCE" for a final price of $30.90. With free shipping, that's the lowest total price we could find by $2, outside of the below mention. It's available in one-size-fits-most.
A close price: Jet.com has it for $25.75 plus $5.99 for shipping.