The way Google did it was that you listed where you were going and what dates and they did some sort of magical formula to figure out your max allowable costs f

The way Google did it was that you listed where you were going and what dates and they did some sort of magical formula to figure out your max allowable costs for the trip. They were always well above the minimum costs. For every $2 below the cap, you got $1 to be able to exceed the cap in the future. It ended up working out so about every 5 economy trips you could do one business class trip (depending on how economical you were). I felt like it aligned the incentives well.