Overstock.com has the Serta 4-inch Memory Foam Mattress Topper + Contour Pillows for $135 + $3 shipping = $138 shipped. Buy a King and cut it in half to have two Twin sized portions. Made from 3-pound density rated memory foam, a bad approximation to Tempurpedic's 5.35 lb density.

Memory Foam 1- inch Mattress Topper = $37

Serta 3-inch Memory Foam Topper with Contour Pillows = $11

Deluxe Memory Foam Accessory Ottoman = $66