City-Green via eBay offers this 15-watt Pet Bed Heater Pad for $12.99 with free shipping. That's the lowest price we could find by $8, outside of the mention below. It measures approximately 12" x 12", is waterproof, and features a 25" power cable.

A close price: Yallstore via eBay has it for $13.79 with free shipping.