5B to match Akainu, but 1 or 2 berries more, maybe some combination that reads like wordplay in Jap?
5,010,111
I'd like to think his bounty is if Akainu went rogue and turned on the WG. Akainu being a former admiral/fleet admiral with such a dangerous power would greatly demoralise the marines, and his bounty would be absurd.
Dragon was in the navy at one point, perhaps he made admiral while aokiji, kizaru and akainu were Vice-Admirals, showing he was a talent among talents, then he chose to leave for one reason or another.
Someone like that who understands how the navy (the first defence of celestials) operates, and is directly going after celestials, the world taboo, is bound to have a crazy bounty. I mean Kuma back in the day was looking up to him, so even decades ago he was influencing Kings and probably turned many into estranging themselves from the WG.
The Celestials need slaves, and Dragon is turning those slaves into freedom fighters, that's why him meeting with Luffy (now joyboy) is so important