One way is to have a jar with $20 bucks in it, and during your tournament, let the participants, if they want to, put $1 in the jar, and if the legally make the 8 Ball they get all the money in the jar, you will be surprised to see how quick the pot will grow. If no one wins it, it just continues on at the next tournament. You will have to check all the racks to make sure the 8 Ball is paced where it is supposed to be.
A bar here in WI has a Fri night $5 entry fee, single elimination 8 Ball tournament, they sell $1 tickets during the tournament (buy as many as you want) all the ticket sales goes in to a continuous pot, at the end of the tournament, they draw one ticket, to break a 10 Ball rack, you must first make something on the break, and then make the lowest numbered ball legally, you get paid for every legally pocketed ball, including any balls made on the break, the kicker is, you can stop anytime before you shoot, and get paid for what you have made, or try to make more cash, but if you miss you lose it all. Last Fri it was at $600 a ball, a guy I work with made three balls for $1800, now it is back down to $400 a ball, the trick is getting your name drawn.