I wouldn't say it's that low. After all, it would be a stretch to say the other Yonko Commanders are much different in bounty to Katakuri, so being more or less 2bil is fine for any of the Yonko. Luffy being close to them makes sense as his massive bounty and being made the unofficial 5th Yonko is mainly a ploy by the World Government to make the Straw Hats a major target and entice big names such as the other Yonko into taking them out for them.