Dell Small Business offers its Dell Inspiron 3650 Intel Skylake Core i5 2.7GHz Desktop PC (no monitor) for $529. Coupon code "GREEN100" cuts it to $429. With free shipping, that's the lowest total price we could find by $121, although it was $30 less in October. It features an Intel Core i5-6400 2.7GHz Skylake quad-core processor, 8GB RAM, 1TB 7200 rpm hard drive, DVD burner, 802.11n wireless, Bluetooth 4.0, USB 3.0, HDMI, 5-in-1 media card reader, and Windows 7 Professional 64-bit (with a Windows 10 Pro license).
Update: Even better, use coupon code "INSPIRON379" to drop it to $379.