Eric Wilhelm, the founder of Instructables (a site that teaches you how to build just about anything you could dream up), has detailed his journey of building a standing treadmill desk for his home office. He actually details how to build three separate desks, running as little as $150 for everything (keyboard platform and desk attached to treadmill, keyboard platform attached to treadmill and desk attached to wall, and both keyboard platform and desk attached to wall).
The instructions are quite detailed and easy to follow (nine steps in all), so if you have the will, some tools, and a bit of free time you should be able to grab a few parts and create your own standing treadputer desk.