The stars were used for fodder marines. I'm guessing Oda will give the marines pirate-like bounties, simply because if not, then what's the point?
The WG already has a power level system of their own, that being doriki. Oda could have just introduced an updated version of doriki that accounts for haki and DFs. But bounties necessitates comparison to the pirates, there's no point in introducing this if the point isn't to compare the admirals and other marines to powerful pirates.
My guess is that Buggy's bounty system and possibly even the WG's bounty system, exists for the sole purpose of power scaling and causing trouble for marines and pirates respectively. And that neither the WG nor Buggy actually expects someone to cash in a 1+ billion berry bounty. For the WG at least, this has to be true, because bounty hunters are all fodder. Anyone strong is either working for the WG or is a criminal. Its probably the same for buggy, who is going to defeat an admiral? The only people on cross guild who can do that are Mihawk and *Possibly* crocodile, who are the true captains of the crew.
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I'd say
Imu ~ 10 billion
Gorosei ~ 2 billion each
Akainu, Sengoku, Kong, Garp ~ 5 billion each
Admirals~ 3-4 billion each
high end VAs(ex. gion, chaton, smoker, etc.)~ 1-2 bil
mid VAs ~ 400- 900 mil