Btween 1.8B to 2.2B. Can't see it being more than BB.
BB has a crew, strong-notorious and most wanted criminals working under him. Is a Yonkou, have territories. Misused Schibukai system to break into Impel Down, starting mass kiling among the Criminals and took the most dangerous ones with himself, allowing few others to also escape in the meanwhile.
Captured Ace, which also led to MF war, technically is the one who killed WB, strongest Men in One piece and stole most Dangerous Gura Gura no mi df. Is the only user with two dfs, and then fought WB commanders in thr war of best and defeated em. Attacked Revo later on in their turf acc to Newspaper report, his crew has been hunting dfs in the 2 years time period.
Compared to that Mihawk is a reasonable ma, who had his bounty frozen for most of his active years as a Pirate if he is. Remained a Schibukai all these years, so probably didn't do anything against the Marines or WG and has feats of battles of Shanks in the past, tho not a Yonkou thn. Mihawk also does cut Pirate crew to kill boredoms, and probably send back hundreds of em.
If Kaidou, Shanks, BM, WB n Roger bounties were only based on Strength, i would also given Mihawk arround 4 B bounty but that is not the case. It factors in lot of things like crew, territories, danger, acts both against Marines n WG, strength and stuff. And so far only strength seems to be in Mihawk favour.
And even with all these stuff mentioned above, BB bounty is still 2.2B, even tho he has so much more than Mihawk. BB probably already at similar lvl to Mihawk if not stronger, crew, territories, feats, acts against WG/Marines, poses great danger since aiming to become PK.
So logically i would have given Mihawk 1.4B bounty, much less than BB at best.
But i also don't see Oda doing Mihawk that way, especially since even Luffy has surpassed that benchmark and has 1.5B bounty. (tbh tho Luffy also has done a lot)
So, my personal number would be 1.9B bounty for Mihawk.