It's not at the first glance only. I did some calculations in PS forums and it seems like Gold star gets 34% effective rakeback, silver 31% and bronze 59%.
Before any1 starts flaming: calculations are based on the amount of each of the prizes according to PP's T&C, so it doesn't mean everyone gets 34% rakeback out of it.
And... we all know how much is 500VIP promo worth on average...