Well, looking at the current arc.... Luffy is about to take on doflamingo alongside law. The implication of that is at the bare minimum a world wide distabilization of everything. Just taking out a shichibukai puts the entire balance of power at risk and is enough for superpowers to make moves based on that one change. Doflamingo is in a worst case scenario a potential match for an admiral, controller of the underworld, king of a nation and the most trusted man in the underworld. If something happens to doflamingo at the bare minimum kaido is to a large extent screwed considering he would have lost his main weapon supplier. A lot of other people actually rely on doflamingo for weapons and such for that matter. Now, with doflamingo being who he is and the implications of him being defeated then why is a 600 mil bounty or even more too much for luffy? Even if the world government is unaware of how unfluential doflamingo is, how much would he be worth if he still had a bounty? Luffy getting a 600 mil bounty for messing with a shichibukai far more powerful than crocodile or moria is not that weird IMO, doflamingo right now should actually be worth well more than 500 mil at least (I would also argue he is quite a bit stronger than what ace was).

I don't think 750 mil is a number which would be fitting for a PK to be honest. I would think a yonko would actually be at least in the 800 or 900 mil mark. WB or dragon probably break the 1000 mil mark considering who they are and the bounties their subordinate have. Ace was not even the strongest in WB's crew and had 550 mil, pirates like marco, jozu and vista, who have been at sea for decades should be well worth more than that.