You clearly do not understand economics.
When I enter into a trade, the amount of wealth I gain is my utility for the item I am purchasing. The amount of wealth I lose is my utility for what I'm giving away. This is true whether we are trading money for goods, or baseball cards for other baseball cards. Money is just a convenient item that people want which can easily be used to keep score. But since we each value the item at a different price than the trade, we can both become wealthier when we trade. It happens all the time. It happens every time I purchase an apple from the store that the shopkeeper was not willing to eat, but which I am.
Let's go back to your example. In your example the person who spent $80 and was willing to spend $100 winds up $20 off wealthier than they were before. That $20 can be traded for some other item of sufficiently higher value, so all the money is likely to remain in circulation.
There are other people for whom the item was only worth $90. They are now willing to buy it. And each one that does winds up $10 wealthier. That's new wealth spread around the world.
The seller, in your example Sony, now winds up benefiting less from each trade. But they still benefit. And they get new trades that they wouldn't have made before. They might or might not wind up wealthier at the new price point. But even if they become less wealthy, the world as a whole is more wealthy.
Incidentally the figures are bigger than you indicate. According to Sony's stock report for Oct-Dec 2000, they were making $175 in profit per PS2 sold. I believe that the Xbox got them to drop the retail price by over $100. At that point Sony was still making a handsome profit. Just less handsome than it had been. (And in truth they didn't have to drop it that much, I think they were trying to make the Xbox too expensive for Microsoft to stick it out. At that price point they made money and Microsoft lost money on every single console sold.)