My daughters play sports at their high school. The big money raiser for those activities is the annual fall car raffle. This year, it's a (new) Nissan Sentra. The tickets are $20 each, which is tough when practically every high school kid in town is selling them; both high schools are part of the raffle and most extra-curricular activities are part of it. Luckily, daughter A in Cross Country only had to sell 5. Daughter B in Field Hockey had 10. I should say they don't HAVE to sell them, but there is a great deal of pressure to do so. We were having our hardwood floors refinished in the middle of that and had to move all of our furniture to the deck and garage for a week. Daughter A's 5 tickets just disappeared, and they must have accidentally been thrown away because they never turned up after moving everything back. I ended up paying for all 5. $100 worth, but at least they put new numbers into the pot so I do have a chance of winning. But ugh, I will be glad when we're done with fundraisers! These 2 (twins) are the last of my 5 kids still at home, and we've seen so many fundraisers over the years it's not funny.
But back to your question. I'd buy whatever you feel comfortable donating to the cause, whether that's $0 or $50 or whatever. If you win, as other have said, you can always sell the car and make some money. But chances are you won't win so figure you're doing it for the cause and not the potential of winning a car.