Too bad you said under $2k, because the curved treadmills like the Assault Runner will get you 2X the workout in 60% of the time (these are purely made up numbers by me, but feel correct). I experienced it before I thought about why it was. The answer is because every step you take pushes an 80lb belt, and while gravity helps on the down step it still requires force/work. You don't "feel" that work until after the workout is over, at which point you are WRECKED.
I would choose a Woodway ($15K) over the Assault, but otherwise, it's the best treadmill workout I've experienced.
(I used to sell treadmills when I worked at a sporting goods store in high school and college, so I know a little more about treadmills than your average Joe, but then again that's been a while ago and technology has changed).