Target offers the Eclipse 42" x 84" Thermaback Blackout Single Panel in Blue for $5.33 with free shipping. (Home Depot and Amazon charge the same.) That's the lowest price we could find by a buck.

Update: Home Depot now charges a few pennies more.

Update 2: Target has increased its price; however, Amazon still charges $5.33 with free shipping with orders of $25 or more.