Been eyeing the Surface Pro 4 but thinking you can do just fine with an old model? Microsoft is aiming to encourage you to upgrade in this situation with its new trade-in program that's said to get you anywhere from $100 to $700 on an old Surface model which you then must toward the Surface Pro 4.
While other websites like Amazon will do similar, the advantage here is Microsoft doesn't ask questions about the condition of your device -- provided it's in decent shape, there should be no issue getting your credit. Plus, it offers a little more than some services (and a little less than others).
The MSRP of the Surface Pro 4 is $899, so you'll need to cough up at least $199.