As the price of he PPV goes up, the number of interested viewers goes down. If you plot all of the possible prices and their corresponding number of viewers, you have your demand curve. The slope of that line is called the elasticity of demand, and indicates how sensitive your consumers are to a change in product price. Two of the points on that line have been demonstraited recently:
1) price 0, viewers 2,000. This was seen in the SVB vs Strickland TAR match.
2) price $25, viewers approx 250. This my own ball park from other recent streams, but I'm sure Justin could give you accurate data.
If you calculate the slope of the line, then given either a price or a number of viewers, you can calculate the other one, and see where you would be in terms of total revenue. If you get 2,000 viewers for a free stream, and 250 at $25, then if it were a $5 PPV, and you calculate that you might get 1,600 viewers for example, then you would take in $8k, which is more than the $6,250 that you would take in if charging 250 viewers $25 each.
After you plot that line (curve actually) you choose the price that you would like to charge. In certain cases you might want only a few customers paying a very high price (ex: Lamborghini or Bentley), in other cases you might want many more customers, who each pay a very low price (ex: iPhone apps, or iTunes songs.) In the case of producing a video, you have primarily fixed costs, meaning for the most part, it costs you the same to produce a video for one person as it does to produce it for 1,000 people. This scenario enables you to keep the price per customer relatively low and rely on higher customer volume to produce the revenue. In terms of a business though, the goal is to maximize revenue, and if you have plotted your demand curve accurately, THEORTICALLY you can calculate the exact price that will yield your maximum revenue.
I realize that there are other factors in play here, e.g.: popularity of the players, Spring being sort of the end of "pool season" (at least here in the mid-west, variable streamer pricing, and on and on, but this is the Econ 101 portion of the price selection.