Xcceries via eBay offers this 48-volt 1,000-watt 20" Fat Tire Bicycle Electric Motor Conversion Kit in Black for $199.90 with free shipping. (Yescom via Rakuten charges the same.) That's the lowest price we could find by $6. It features a thumb throttle with LED indicators, power brake, and pedal assist sensor.

Note: It doesn't include a battery or charger.