Yapper Wireless via eBay offers the Google 18-watt USB-C Wall Charger for $19.99 with free shipping. That's the lowest price we could find by $15, outside of the mention below. It includes a 5.9-foot USB-C cable.

A close price: Groupon offers it with a USB-C to USB-A cable and a USB-A to USB-C adapter for $19.99 plus $3.99 for shipping.