Luffy's bounty is gonna be 4.5 B at least and < 5 B, at the end he will defeat Kaido and have subordinates, that took Kaido's commanders down and have a huge alliance that surpass that of any other Yonko, he will have the control over Wano and the WG will lose the weapons supply. Luffy's either get 3*former bounty or little bit higher or he gets +1 on the first digit of his bounty.
so either 2.5 B or 4.5 B+
2.5 is definitly two low when 2 Yonko's go down, Luffy an alliance with people like the minks , the samurai , grand fleet, Marco and even rivals like Kid and Law, who possibily will use team work to defeat BM and have 2 bounties , which their sum should be higer than BM's. somthing like 2.2 ~2.5 for each of them or you could see around BB's current bounty.
BB didn't defeat any yonko but some Yonko commanders like Marco with bounty of 1.37 B and just took control of some of the territories of WB.
Luffy would have taken a Yonko , his alliance taken the other and more than that he will have control of Wano, the place the WG wants to be closed for ever.