Is a person allowed to pray for something that is statistically very unlikely to occur, such as winning the lottery?
We learn in Brachot 10a - from King Chizkiyahu - that even if the executioner's sharp sword is already at your neck, you should pray for mercy.
אפילו חרב חדה מונחת על צווארו של אדם אל ימנע עצמו מן הרחמים
What are the chances of surviving an execution when you can already feel the cold metal on your neck? Yet we are taught that even then we should pray.
That said, it's usually more efficient to pray for riches rather than to tell Gcd how to make you rich!