Everything = The collective amount of however much is wagered in that 12 hour-ish time frame. If one person bets 1k, and nobody else bets, the jackpot is 1k, and since he will have the closest number to the real guess, he will get the 1k he wagered. If another person bets, and 2 people bet in total, then someone will get 2k from the exchange. Very simple.
The point is I make 0 profit, and the more people particpate, the more there is in the jackpot, and there more you put in the jackpot the better your odds of winning.