Just noticed this:
Marshall D. Teach - ฿3,996,000,000
Dracule Mihawk - ฿3,590,000,000
Buggy - ฿3,189,000,000
Trafalgar Law - ฿3,000,000,000
Crocodile - ฿1,965,000,000
Boa Hancock - ฿1,659,000,000
Jinbe - ฿1,100,000,000
Seeing as how 7 warlords so far have gotten bounties over 1 billion...