Since Roger is at 5,564,800,000 Billions, Rayleigh might be in the 4 Billions range especially considering that Mihawk is roughly at 3,6 Billions when he is far less accomplished as a pirate than Rayleigh
Whitebeard and Roger were considered equals, but the Roger pirates in general have higher rep due to the fact they are the only pirate crew who reached Laugh Tale, and thus naturally Rayleigh as the first mate has higher rep than Mihawk for this reason
I won't be surprised as well if part of his rep comes from the god Valley incident where the government tried their hardest to diminish pirates accomplishments(especially as Rayleigh being a subordinate who is less highlighted than captains in the news) in favor of the marines, especially Garp. It could also explain why Garp knew exactly how dangerous he was when highlighting the threat to involve Rayleigh in the MF war