It depends if the Marines only re-impose them or re-impose them and increase them. If they only re-impose them i don't see anyone to have above 1 billion.... but if they re-impose and increase them then it's a different story.
Weevil's bounty I believe was frozen at 450 million and Luffy's after dresrossa was 500 million so I can clearly see Weevil being 1B+. other 1b+ would be Mihawk, Boa, Doffy and Buggy. But I do agree with you that people should temper their expectations etc. Don't expect Mihawk with a 3 Billion bounty... your just gonna be setting your self up for disappointment.
The guy isn't really a direct threat, has no crew nor territories at all and lives in Paradise.
Teach is an emperor with territories, thousands of subordinates, a fleet of a dozen main ships minimum, the owner of two of the most dangerous fruits in the world..., yet he's barely above 2 billion.
It wouldn't really make sense for Mihawk's head to be too expensive. The one billion range is actually reasonable considering his context, because sure, he's got the strength, but he lacks in literally anything else that the characters with 2b+ have. How can he even dream of having 5 billion, as another user suggested, if the only pirates to have ever reached that were the factual and virtual kings of the sea? It doesn't hold up, I think he will be very lucky if he reaches the 2b mark by himself. Something around 1-1,5b, 2b max seems more likely to me.