Something I've seen a few times since last year and that I think works pretty well are the point based prizes because it serves 2 purposes.

First if the expected sales aren't met then the affiliates don't have enough points for the big prizes.

Second all affiliates have a chance for a cool prize, so it's not the same big guy(s) getting the first places all the time.

I do favor the "prize" model rather than cash because some times it's fun to send out that extra email and win something you think is cool but you regularly wouldn't purchase for yourself.
...and in the end the affiliate can chose for the prize or it's cash equivalent

my 2 cents

Cheers,

Richard