I would consider the ThinkPad E550 which has a base price of $579. It comes with a Haswell generation Core i3-4005u @ 1.7GHz, but the new Broadwell generation Core i3-5005u @ 2.0GHz is a free upgrade which is not only more powerful but also consumes a little less power. However, I would upgrade to the Broadwell generation Core i5-5200u for $100 which has a base clock speed of 2.2GHz and a max turbo boost speed of 2.7GHz.
http

/shop.lenovo.com/us/en/laptops/thinkpad/e-series/e550/
It only comes with 4GB of RAM, but you can upgrade to a total of 8GB for $80. Or you can simply buy a stick of 4GB PC-12800 DDR3L 1600 1.35v RAM for around $40 or so and install it yourself. The screen is only 1366x768 which is too low for productivity...