He doesn´t have any impressive subordinates, territory, poneglyphs or political alliances. His piracy career consists of attacking random government ships and killing Harald if you want to count that. 3-4 billion should be fine
Loki war probably fine around 2.6 for what he is known for and his threat level. Now, if he becomes King of Elbaph and gains Dorry and Brogy as his #1-2 + all other semi-impressive characters except Hajrudin with crew he enters 4.0-4.5B territory. Ad his DF and he might enter 4.5-5B territory.
Mihawk has a 3.5B bounty despite having even less than that and being much weaker than Loki, so 4B would be the minimum
Loki has been very impressive but Mihawks feats
and portrayal still holds up. No scars, no defeats. One man army that hunted Marines, reached WSS 20 years ago, wielding the worlds strongest blade that he also turned black and whose only rival was 2 armed Shanks.
There are reasons Mihawk, Shanks, Dragon, BB, Akainu are still rather unknown battle forces that we only have seen glimpses off. Those 5 are all 40-55 years old, absolute PRIME in OP terms.