You could calculate the surface area of the ball and then look up the local prices of fiberglass mat online. That would give you an accurate estimate of how much you'll need. One layer of mat should be enough for the prop, unless you plan on putting weight on the ball or throwing/rolling it around.
Paper mache will be cheaper, but the prop's going to be noticeably heavier than fiberglass. If you don't need to lift the prop, I suppose it wouldn't make much difference. But you will sacrifice some durability with paper mache.