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She has a bounty of 80,000,000 after the CP9 saga. She got a 79,000,000 bounty when she was 8 years old, for her ability to read Poneglyphs.

It should also be taken into consideration that her 79,000,000 Belli bounty was issued when she was only 8 years old, the World Government must have reached the conclusion that this amount was sufficient for a pre-pubescent girl - albeit in possession of devil fruit powers.

Yes, the 79 millions bounty was given when she was a child. They thought, maybe, that it would be easy to capture her, so why give a higher bounty? Marines don't give money for free! After some year, probably working in Baroque Works, she "desappeared" as Robin to become "miss all sundays", a girl with no bounty. So Robin's bounty didn't increase. In the CP9 saga she did few things; her bounty grew only because she's an adult with a dangerous DF, but her ability about poneglyphs was not involved. In the future, if she discovers a new ancient weapon or similar, her bounty will surely grow up.

isn't she already did? what about the poseidon in sky island?

As far as the outside world is supposed to be concerned. She was supposed to be dead. So them giving her a real high bounty would have basically been like a headlighted sign with neons saying "WE FAILED" on the World Government's part, and her escaping Enies Lobby and Buster call twice was already enough. Giving her a high bounty would of just been like the World Government saying, this woman has bettered us, so they placed most of the blame on Luffy, because he's new to the scene so they could act like he's been under their radar. Least in my mind. I think too much. :P PDT.