Interestingly enough, there was an experiment done regarding a similar situation. They told a group of children that if they played a board game for an hour, they would each receive something like $15. They went off and played the game for the full hour and nothing more. However, another group of children were just given the game and went well beyond an hour.
Conclusion: Rewards distract from the actual benefit of the activity.
However, since I know you're not debating how well this works for getting them to read and just want ideas because you're in charge, I'll ask you to give us some more details. What's the reading for? In what environment is this? Is this required reading or extra reading? Is it a class prize or reward or is it only for each individual person?
I can't come up with a good reward if I don't know the circumstances. Pizza is a great reward for a class, but give one kid a whole pizza for reading and you're spoiling him. If you could outline the details it'd help us out a ton. Will help as much as I can.