I think he had a zero bounty, that's why he was looking for a big ass pirate to take out (to make a name for himself), I remember other pirates (SSB mostly) asking who he was before meeting him.
we can probably use Marco (one of marco's first officers) as an compartment bounty. my guess is that ACE has around 150-200mil bounty. since he belongs to WB and the threat he posses to the government is accumulated into the general bounty of WB himself.