Joseph – to clarify
When you purchase a $100 raffle ticket toward a charity, there are two parts. ~ $5 goes toward your own chance to win the prize. (This based on your odds of winning) Your money, your prize. The ~$95 additional is the amount donated.
If you win or lose – Deduct only $95 from your Maaser distribution account.
If you win – pay 10% Maaser on the full winnings of $3000.