Instead of having the redemption rates jump with each time someone turns in 3 cards like so.
4-6-8-10
How about making an option for each player to have it like that...for example, lets use players x and y
Player x has redeemed 12 cards in total, his next cash in would be 12
4-6-8-10-12
Player y has only redeemed 9 cards, his next cash in would be 10
4-6-8-10
This way it isn't just luck by which you get more or less men you think you might and you know what you will get. I think this would be slightly better then flat rates cause it still escalates but at the same time requires the player to be aggressive throughout the entire game to get more and more bonuses.