According to a retailer, it'll cost $59.99 & wil be available in 2-3 weeks:
http://www.operationandroid.com/content/nexus-4-wireless-charging-orb-pricing-release-info-revealed/31313092
http://www.operationandroid.com/content/nexus-4-wireless-charging-orb-pricing-release-info-revealed/31313092