The Dell Optiplex 7040 Micro miniPC is available on Amazon for $619.99 + $19.99 shipping ($639.98 total). Cheapest ever!
This little business miniPC is tough and solid. It includes a rather powerful 35W quad-core i7-6700T CPU, 8GB 2133MHz DDR4 RAM, a 2.5" 128GB SATA SSD and Windows 10 Pro.
On the back you'll find one HDMI 1.4 port, one DisplayPort, 4x USB3.0 and a Gigabit LAN port. Seems like it could also serve as a great living-room HTPC.
You also get a Dell USB Keyboard & USB Optical Mouse.
As the following shot I took shows, there is an option to add one more RAM stick and an M.2 SSD:
Covered by Dell 3-Year On-Site warranty.
*Please note that this particular model does not include a WLAN card.