Brook is definitely not a nobody but you have to look at his known threat level.
Inflation happens but compared to what Brogy and Dorry did, Brooks crimes amount to much less which wouldn't warrant an even higher jump. 100 million over 100 years ago is no joke. 100 million in our world in the early 1900s is a ton of money and my math sucks so you won't see much calculations from me but even I can see that there needs to be an adjustment to make that 100 mil equal to what 100 mil should be currently. And then they slashed Mr wormgorosei's head off. Brook was there, not discounting that fact but what did he do really? Maybe I need to reread the egghead arc again but he didn't do anything to show that his threat level went up.
As I've said before he wrestled Big Mom's personal homies and can even hurt them AND stole the rubbings. The WG doesn't know any of this. If they did then there may need to be further adjustments. But then again he likely would get a raise in bounty just by being there on Egghead as part of a yonkou crew, we just haven't seen it yet. To us, we know he is a threat but to the WG, he isn't that bad. I actually don't think everyone should get a bounty raise. The reason why is because there are 4 yonkou crews. Everyone fights or does a part but not everyone is recognized. Especially chopper. How can everyone be recognized? It isn't possible even if the yonko crew consist of just 2 handful of people. The badass ones just keep taking the spotlight because they are doing stuff like blocking hits from horsemen gorosei or Sanji not breaking a sweat vs S Shark. Even Bonney socked the shit out of Saturn. Imu is watching. He saw how the giant fodders were able to block Saturn's poison leg strikes. He did not see Brook do that because he didn't. To be fair, I would say the WG would treat Chopper, Nami, Usopp, etc the same. So the increase would not be much. Now I'd expect them to increase the shit out Zoro and Robin's bounties though because their threat level kept increasing. If Zoro and Sanji went away for a hundred years and came back and one of the first things they do is cut off a gorosei's head, I'd expect a massive increase too. Like holy hell, not only are they not dead, they came back with a vengeance.
Here's probably one of the best examples of bounty relating to threat level: Blackbeard. He went from nobody to being the big man on the block and his first bounty, once the WG recognized his threat level was 2,247,600,000 berries but that is because the WG knows his exploits. Prior to that he was a bounty less figure on WB's ship and then at some point he injured Shanks and then defeated Ace. Let that sink in a for moment. The WG only hands out bounties based on the info they have at hand. Had the WG known of such capabilities he might have received an earlier bounty. So, Let's just say Brogy and Dory didn't even have their original 100 million bounty. It honestly doesn't even matter. Giants = Known to be strong af warriors. And these are the captains. They decapitated a gorosei and resisted others. Then they "killed" VP and saved another VP along with Robin AND Luffy. Now they get to live and tell the tale back to their homies on Elbaph turning all of Warland against the WG. Not to mention them fucking up a bunch of marines and battle ships in the process. Even Saturn jumping on their ship wasn't able to do anything. They made a fool of the Gorosei and the marines all saw. What bounty would you give that type of threat even if these guys never had a bounty? Me personally, I'll be a little conservative and give them 750 mil because all of this takes place in one umbrella event (egghead) but 1 billion does fit the bill here. Not many people can wander into an island during a buster call and come back alive and basically unhurt in the face of one hell of a marine force.