Disillusioned: Where did I say that the $3,000 win must be given to maaser or that the winnings belong to maaser? I said no such thing.
You’d have to give maaser on the winnings; if you won $3,000 after spending $100 at the mosod’s auction, 10% for maaser on the $2,900 net winnings would be $290. At that point the original $100 is no longer considered to have been given to maaser, since you got it back, so you’d have to give that $100 to maaser (since it presumably came out of your maaser cheshbon.)
Another valid approach would be to consider the original $100 to have been given to maaser, despite your winning a larger amount. And then give 10% maaser on your $3,000 win ($300).