Thats also partially because the Funk Brothers are technically bounty hunters despite being criminals themselves.
While they may be criminals, it is their job to hunt criminals, and as such, aren't considered a terribly active threat. At most, they committed some petty crimes.
This isn't the same for someone like Zoro, who is an infamous pirate who was involved in a crew that caused several major events.
I don't see why Pica's bounty would be above Zoro's, much less above the bounty Luffy and Law recieved for defeating Doflamingo.
While they may be criminals, it is their job to hunt criminals, and as such, aren't considered a terribly active threat. At most, they committed some petty crimes.
This isn't the same for someone like Zoro, who is an infamous pirate who was involved in a crew that caused several major events.
I don't see why Pica's bounty would be above Zoro's, much less above the bounty Luffy and Law recieved for defeating Doflamingo.
Pica's bounty can easily be above Zoro's, or Luffy's and Law's, if he has been a seasoned pirate who reached his full bounty potential, like some of the Yonko crew members, Pekoms or Tamago for example. As stated, bounty doesn't work like: X beating Y, so X gets higher bounty than Y, Luffy, Law, Zoro were all much stronger than their bounty numbers after DR, they did things, so their bounties went up a bit, simple as that. They aren't good reference points, if you look at Sabo's bounty, there's no way somebody like Jack who should have 1Bil bounty, but he does. But if you look at somebody like Bellamy, Jack's 1 Bil bounty is justified.
If you try to justify Pica's 500 mil bounty(minimum) with Luffy, Law, Zoro, of course it doesn't make sense, but there are many many examples who can justify this.