Your point about there not being a difference between 5000 and 3000 is simply wrong. It's different a factor of 5/3. How many billionaires would turn down the opportunity to have 5/3 more money? None.
Well yes they didn't get to be billionaires by turning down money but the more relevant question is how different would their lives be with 5/3 more money.
Depends on what they do with the money. If they sit on it, nothing. If they try to maximize their ability to improve the world with it, then they can make 5/3 as much of a difference.