GearBest offers the Children's Car Seatbelt Cushion in Off-White for $2.35 with free shipping. (It's available in several other colors for a few cents more.) That's the lowest total price we could find by a buck, excluding the mention below. It's made from cotton.

A close price: Allstar-seller via eBay has it for $2.84 with free shipping

Note: This item ships from mainland China and may take three to four weeks to arrive.