How about this? The goal is to make $10,000, but the player wins if the total is anywhere between $9000 and $11,000, inclusive.
The spiel: "I want to give you ten thousand dollars in cash and prizes. The problem is that the prize I want to give you has a price of less than ten dollars thousand. Write the check for amount that will make total of the prize and the check for ten thousand dollars. If you think the prize is one thousand dollars then write the check for nine thousand. One thousand plus nine thousand is ten thousand and you would win! If you come with one thousand of ten thousand, high or low, you still win. Now, get writing!"