So what we know of the bounties of the Yonko and their commanders is this: Blackbeard has the highest bounty at 2,247,600 Berries and Yonko Commanders (Queen, Katakuri, Jack, Smoothie) have a bounty in the 1-1.5 Billion Berries range.
Granted, the Big Mom Commanders mostly stay near Whole Cake Island, so the Marines aren’t necessarily aware of their power levels or capabilities (e.g. Marines didn’t even know that Cracker wore a mask).
Whereas Jack, King and Queen go around destroying anything and everything (hence the name “Calamity”), and are thus probably given a slightly higher bounty than the Big Mom Pirates due to the sheer collateral damage they cause.
Now let us assume that King’s bounty will still be around 1.5 Billion Berries (roughly Luffy’s present bounty) since he has been hinted at being more powerful than Jack and Queen BUT most likely doesn’t have a higher bounty than Teach (a Yonko).
This accounts for him being more powerful than Jack and Queen but also not having a considerably higher bounty than the more low-key Katakuri. And if we assume that Teach has the lowest Yonko bounty because he’s the most recent addition to the Yonko, we can assume that maybe the remaining Yonko bounties are in the 2-4 Billion range. This would also hint that Marco, Benn Beckman and King have bounties of less than Teach’s 2,247,600 Berries.
Gol D. Roger, being more powerful than the present Yonko, should have a bounty of even higher (possibly around 5 Billion Berries). Since Luffy has been hyped up to be even more powerful than Roger at the end of the series, and since we know Oda likes to play with the numbers 5 and 6 because they can represent “Gomu” in Japanese, maybe Oda will make Luffy’s final bounty as 5,600,000,000 Berries.
I think this is plausible considering Oda’s playfulness with Luffy always being around the numbers 5 and 6 and the power scaling we have seen so far from Jack, Katakuri, and Teach. Examples of Oda playing with Luffy and #56:
Luffy’s number in the colliseum at Dressrosa: 0556
Number of people in the Straw Hat Grand Fleet: ~5600 (might end up at 5656)
Katakuri’s bounty: 1,057,000,000 (hinting with the “57” that he’s slightly better/more powerful than Luffy).
Barto Club (Bartolomeo’s fanboy crew) having exactly 56 members
*Theory by Dynastian98