Groupon offers the Furhaven Orthopedic Medium Pet Mattress in 4 colors for $16.99. With $3.99 for shipping, it's the lowest total price we could find by $2. It features a zippered cover and water-resistant bottom.
Of note, Groupon also offer it in Large or Jumbo for $24.99 and $34.99, respectively. With free shipping, each is a price low by at least $13.