Does anybody know the density of the Polyscience or VS balls? Each ball could potentially remove an amount of water surface area equal to its max cross sectional area if the density is half that of water. Assuming dense packing, that would be 91%. Ping pong balls are very light (2.7g, 40mm diameter), and only a small portion of the ball would penetrate the water, so a large portion of the original surface area of water remains. By my calcs, ping pong balls would only remove 52% of surface area, which might result in less efficient insulation relative to a heavier ball. On the other hand, the balls themselves might reduce the airflow at the water surface, and slow the evaporation that way. Without actually testing it, I obviously can't say how large an effect this would have, or if it would fully cancel out the effects of having a greater surface area. I'm not saying anything definitive; I just wanted to bring up the possibility that the Polyscience/VS balls are optimized for the task, and perhaps this is the reason behind the exorbitant cost. I'm very curious why a bag of plastic balls would cost $100! Caveat: It's late right now, so I don't fully trust my calculations to be perfect. And there are lots of effects that I am ignoring, such as surface tension and possible hydrophobic ball surfaces.
