Amazon offers the Cosco High Back Booster Car Seat in Ava, model no. 22253BMO, for
$32.64 with free shipping. That's the lowest total price we could find by $7. This forward-facing car seat is designed for children weighing 22 lbs. to 40 lbs. and 34" to 52" tall. It also can be used as a belt-positioning booster seat for children weighing 40 lbs. to 80 lbs. It features a 5-point harness and removable cup holder.
Update: The price has since dropped to $31.53. Free shipping still applies.
Cosco High Back Booster Seat 22253BMO is also available at these stores