Luffy's bounty always triples, or slightly more than triples (Croc). Luffy was 30 million, he beats Croc, he goes to 100 million. That's 3.333 times higher. Luffy now is 1.5 billion, he beats Kaido, he goes to 5 billion. That's 3.333 times higher.
There is a reason Whitebeard's bounty is just slightly more than 5 billion. It's symbolic that even after Wano, only Roger/WB (PK level guys) are above him.
Or he might go to Kaido's 4.6 billion, and then get a 1 billion increase after Elbaf, to reach the holy "5.6 billion".