How much control do you want? How indepth and hands on do you need to get? The Mackie Control is a good little controller. It is not a "Pro Control" for Nuendo, but it works well. That would be as "cheap" as I would go. Anyone else?
I use a Yamaha O1V as a control surface for Nuendo. Works great, and you can link 2 of them together if you need more than 14 fader controls, but you'll also need a USB - MIDI interface for it. The only drawback is you're draggin' around a bank of pre's that'll probably never get used.