It's a good question what it should cost. The more it costs, the less people will buy it. But I guess there's no shipment costs or something like that.
If it costed ten dollars, perhaps 100 would buy it. But if it costed 20 dollars, perhaps only 50 would buy it. Then it would be the same. I don't know where the balance point is.
I can say that I would pay as much as normal computer games cost. Perhaps I would pay 50$, but I wouldn't do that if I didn't know the program.