This Slumber1 8" Mattress-In-A-Box drops as low as $89.98 right now at Walmart.com. This price is for the Twin size (regularly $149), or choose the Queen which drops from $209 to $149.99 or the King size which drops from $249 to $169.99. This is $39 under the previous price low from Memorial Day and the lowest price we found on any size of this best-selling mattress. Sales tax is charged in most states.