It shouldn't be too high nor too low.
For example:
1st Prize = Ticket Price x30 ---> 1,000,000 Zeny x30 = 30,000,000 Zeny
2nd Prize = Ticket Price x20 ---> 1,000,000 Zeny x20 = 20,000,000 Zeny
3rd Prize = Ticket Price x10 ---> 1,000,000 Zeny x10 = 10,000,000 Zeny
Just a first suggestion. Prize are definitely worth to participate, due to the low amount of participants ( few players on server ).
If you wish to increase, then suggest another amount. Just a first step to get closer for a fair prize.