As a reader discovered, Adorama via eBay offers the Microsoft Surface 3 10.8" 64GB Windows 10 Tablet, model no. 7G5-00001, for $279.99 with free shipping. That's the lowest price we could find by $120 and the best deal we've seen for a new one. (For further comparison, it's $20 less than Microsoft charges for a refurb.)
It features an Intel Atom x7-Z8700 1.6GHz quad-core processor, 10.8" 1920x1080 (1080p) LCD, 2GB RAM, 64GB flash memory, 802.11ac wireless, Bluetooth, and Windows 8.1.