As far as I know, Power level doesnt equate your bounty.
Luffy with 300 bounty worth to get thrown into Level 5 Impledown.
Daz Bones is a bounty hunter from west blue this reputation alone worth 70+ Millions, his connection with Crocodile and Baroque Works make him worth for Level 4.
Daz Bones Influence is diffrent with Crocodile in the underworld. Being Crocodile's dog doesnt mean anything when Sabo's bounty only 500 Millions.
Well, Luffy wasn't really thrown in to level 5. Luffy was presumed killed by Magellan and was just going to lay there in level 5 until he eventually would have been buried or more likely thrown in to the fire in level 4.
While true power level doesn't always equate bounty it mostly do. Kaido and Big Mom for example were way bigger threats to civilians than Whitebeard and they unlike him were actively trying to get the One Piece thus a threat to the WG yet he was the one with the highest bounty.
While I agree Sabo is a far worse threat than Daz Bones the reason I believe Daz can get 500+ is because of:
1 - Bounty inflation. When Sabo was introduced he was the highest known bounty in the series. Today it's a shitty bounty, all of a sudden Jack and Sanji are 1B+ with Kidd and Law at 3B. Even Bellamy managed to get 200M when he came back despite not even being stronger than Dellinger.
2 - Retcon/Development. Like Lucci and Kaku - I expect great things from Crocodile and Daz Bones. Their bounties indicate Crocodile to be somewhere around YC1-Admiral range. I could see him take on Green Bull. Daz Bones if it comes true is in the Tobi Roppo-range, an experienced and strong pirate but not at commander level. If Doflamingo returns it wouldn't surprise me if he is 2B and takes on Fujitora, there is already bad blood and he did think he could do it (although I see Fuji win).