Mihawk's bounty will surpass Teach's current bounty from narrative perspective, let alone Ben Beckman's.
Using Teach's current bounty as a standard to explain why Mihawk wouldn't get a higher bounty is practically misleading. Teach may have one of the four most powerful pirate crews but he himself is still not even at half Kaido's bounty, moreover his growth is heavily linked with Luffy, therefore saying he shouldn't cross 2 billions is similar to saying "Mihawk won't get past 1.5B since he will surpass Luffy's bounty" both Luffy and Teach will receive bigger bounties later on.
Logically speaking, there are four things which determine a criminal's threat according to the world government:
1. Strength
2. Achievements
3. Influence
4. Forbidden Knowledge
Depending on the individual he can have one, two, three or even four (in case of Roger) of these standards going for him. In Mihawk's case, there are no hearsays/propaganda roaming around his portrayal like other supposed top tiers, he's a solidified top tier renowned as the world strongest swordsman who kept challenging every swordmaster untill he reached the top and couldn't find any more peers. his clashes with Shanks are admired by Whitebeard himself, but what really made him stand out is being perceived as the unrivaled swordmaster. therefore it's strength coupled with achievements that's going to boost Mihawk's bounty, while for Shanks, it's strength and influence.
Hence why i expect Mihawk's bounty to be at bare minimum 3 billions. story wise, you can even see that the marines are as much worried about dealing with warlords threat as they do with the alliance of Kaido and Big Mom.
