It's funny how your examples only focus on Luffy, Kidd and Law, knowing full well that Luffy's bounty isn't accurate purposefully and that Oda's constantly trying to force in a supposed rivalry with their bounties, it doesn't change that we know bounty is at least a reflection of
- Strength
- Crew
- Influence
- Territory
The Admirals only have the first, therefore their bounties are automatically going to be considerably lower than most relevant pirates who have multiple of those things.
As for Kidd's crew being stronger, no, Law himself is stronger than Kidd and as we've seen recently Bepo, Bart, Shachi and Penguin can hold their own, Law's crew is stronger than Kidd's crew at this point.
Mihawk being a part of a Yonko's crew should give him a pretty good boost actually, and you can't say it couldn't be 1 billion or whatever as we have zero idea what Mihawk's former bounty was.