Hmmm. Well, it's kind of depends upon what exercises you are doing. When I use a med ball I usually reach for a 8lb if I'm doing something that requires more "control" of the ball. Like if I'm passing it in the air around my waist, or doing figure eights with a full squat at the bottom.
Below is a video link to a med ball workout. If I were going to do this one...and I should also admit, I haven't done this one yet I just found it for demo purposes, I would probably use an eight as well.
http://www.youtube.com/watch?v=BwtaE...eature=related
You may also want to see if there is a sporting goods resale shop. I know that when I was purchasing equipment for myself I haunted the "Play It Again Sports" places and could find a few deals.