Trying to find sense in bounties post Jack is impossible.
Like in ID lvl 5 was made for +100m bounties, that are basically the average bounty in the New World. Also the Warlords bounties from pre-TS make no sense at all, Kuma was in the revos for a decade and his bounty didnt reach 300m, while a lot of emperors low-lvl officers have bounties higher than 300m.
The only explanation was if the WG was trying everything to keep the revos bounties low as a way to no give them free publicity and avoid giving the idea that they really are powerfull enough to opose the WG, as we can see that all the revos commanders have really low bounties.
Also, nothing is said that Dragon will have a crazy high bounty, of course he probably have one on pair with the emperors, but dont see it going to something like 4b