How can I tell how good of a workout I got?

Right, so for a science project for school, I'm testing which gives you a better workout, Wii, or Kinect. My first idea was to use a heart rate monitor, but the only ones I can find or 60-70 bucks or more. Does anyone have any other ideas to keep track of the workout? I can't make my desicion based soley on perspiration. I really need help on this one. Thanks in advance!