So in Wano, the alliance of Kidd, Law, and Luffy defeated Kaido and Big Mom.
Kaido's bounty was: 4,611,100,000 bellies
Big Mom's bounty was: 4,388,000,000 bellies
Big Mom + Kaido = 8,999,100,000 bellies
Luffy + Kidd + Law= 9,000,000,000 bellies
As you can see Luffy, Kid, and Law's combined bounty is roughly the same as Big mom and Kaido's combined.
If we apply the same logic to the Dressrosa alliance then Doffy's bounty should be roughly at 1 billion. because:
Post-Dressrosa Law + Post-Dressrosa Luffy = 1,000,000,000 bellies