I'm assuming you mean prize, not price. But for a prize, what I would ask is why not give them a cash egg of their choice? Since they're all the same anyway, the winners will get their egg, but will not be able to choose its size. It's fair and you can do multiple prizes as well. For instance, have an egg for first place at guaranteed 4/5 or 5/5, second place as any egg of their choice, but it's at their risk for the size. Third place could be like 3 evos. I think this works because if the 1st place winner gets a cash egg of their choice, then it's a really good prize. 2nd place can choose and risk getting a 3/5, which helps because if you buy the eggs for first place, and they become 3/5, you won't be wasting money because you already had the egg. Finally, the third place could be evos or something. I don't think third place should really be chips or something really useless. Maybe have it so that they can get evos... A substitute prize could be an expansion package for 1st place, 2nd place gets a cash egg of a certain digimon(but they get the risk of being 3/5), and 3rd place can be evolutors or experience boosters or whatever they will be coming out with.
Event suggestions are posted in one of the topics here as well... I don't sem to remember the topic, but I know there were some really good ideas in it.