Pay as much as you want: I had posted the exact same detail a while ago, but I like it more this time. You’re free to pay as much or little as you want, but pay more than the average to receive a bonus game.

Pricing: The Humble Bundle (@humble) is generally free. You may choose to pay what you wish and you may choose who is going to get how much of your money. But if you pay more than the average for it, you will get an extra (and the average should grow steadily at the same time). 

Realtime social proof: The Humble Bundle (@humble) used a real time bundle sales counter where you could tell that the product sales like hot cakes (along with a time indication to let you know how long you had left to profit).