Senyx Direct via eBay offers the Tri-Fold Nylon Sleeve Case for 13" / 15" Macbook Air or Macbook Pro with Retina (pictured) for $4 with free shipping. That's the lowest total price we could find for any such nylon sleeve by $2. It features a front pocket and is padded for more protection.

Also available is the Esme Dual-Fold Nylon Sleeve for 13" / 15" Macbook Air or Pro for $4 with free shipping. That's another price low, this time by $2. It features a smaller front pocket for a tablet or other accessories.