One thing you will always find in my experience is there is at least one member who is just a whole lot better than the rest. This member will clean up most times in premium tourneys, doesn't matter what the entry is, big or small, you will see the same names at the top of leaderboard.
As tiresome as it is, the only way around it imho is to have tiered tournaments. You will have to be strict about punishing high tier players entering lower tier tourneys, as I am not sure you can prevent it, other than giving everyone fair warning what the penalty will be.
I would suggest starting low at say 10 credits, then gradually increase until u see a drop off in the entrants. And have as much variety as possible.
1i
Edit a fun way to offer prizes in some, not all tourneys is to give say a sleeve for whever finishes in say 8th position as per the WGT rules, in other words if 4 players are tied 8th, then the first persion in that tie gets the prize.
This is a nice way of giving out prizes , and not just having the best score count all the time.
Ur gonna need deep pockets.