This alone makes me wonder why Luffy's bounty isn't higher. He's an extremely huge threat, and he deserves a higher bounty to represent that even if his full strength doesn't match that. Unlike Robin, the Marines have a great reason for giving him such a high bounty in that he invaded Marineford and freed a pirate, even though one actual reason why they want him dead is because of his potential thanks to his Conqueror's Haki.
Luffy's current Bounty is fair & honest.
Before however Luffy's bounty didnt increase very much despite being involved in several incidents...asode from what Jimbei stated , that it is just hard to raise ya bounty after 300k; Here's my opinion why in Luffy's case and perception of why this is overall...
SH vs Cp9 & Eines Lobby = Direct assault on WG institution & high lv Agents. 300K bounty....Luffy proved he could defeat Lucci & thousands of Marine soldiers, plus the act itself..
Luffy vs Moriah...Is kept secret, Moriah also proved to WG he wasn't a big shot, despite them probably overestimating at.....WG isnt threatened by his lost to Luffy.
Luffy breaks in & out of Impel Down + intervenes in the Marineford War......WG cant surmise how he broke in, but his break out wasn't a solo act, but a group effort of many prisoners even lv6 prisoners..Also Luffy's efforts in the war were successful because of the support from the vets around him, Luffy himself couldn't contend with any real threat that was there.....So therefore his bounty only increased 100k..400k Luffy trains for 2 yrs..
400k Luffy PTS is a bounty reflective of Luffy's charisma, luck etc not his power...Other Pirates with 400k bounties are NW Pirates who can use haki & are stronger than this era Luffy.
Luffy 2years later with haki training sporting a 400k bounty in all actually was 500k+...SH ran through Fishman island & Punk Hazard threats with little to no issue..Duffy's gang vs Luffy & co. only alotted Luffy & Law himself another 100k increase......Joint effort to take out a strong captain of a huge crew, Doffy was strong but SH didnt accomplish the feat himself...& info of Pirate fleet isnt public knowledge..So rated on strength World Government still isnt all that worried.
WCI changed the game. News was spun to make BM look bad & Straw Hat look really good. 2 commanders down & Pirate Fleet of 5k revealed...With all the hype a New yonkou was crowned......This bounty is hyped however 2 big name pirates were defeated & bigger stronger crew is being a amasssd..That calls for 1b+
Long winded I'm sorry. But that's my perspective