We've seen with many Warlords that WG doesn't Update Bounty of Someone They want to Recruit to join Shichibukai,
It's not like They gave Blackbeard a New Bounty for Defeating Ace & then accepted him as New Warlord, or gave Buggy a New Bounty for Impel Down/Marineford Act & then Invited him ... etc
Thus when WG were considering giving Mihawk a High Bounty, that's when They asked him to join Warlords & since He said Yes then his Bounty remained Low just like every Single Warlord, no need to make an Exception of Mihawk
So Most Warlords remained with their Initial or Second Bounties which are almost always very low, you're most probably gonna Start with a Bounty Lower than 100M or Hit a Supernova Bounty, even Fisher Tiger who literally Attacked & Burned Mary Geoise, freeing so many Slaves only got 230M as Initial Bounty for that Act
If you consistently commit Crimes & Number of Bodies that you leave becomes too big, then your Bounty can near 500M such as Law & Weevil & Big Mom ... etc We also have an Example of Bounty you can expect for Opposing WG directly, as Sabo had 600M for being No. 2 of Dragon & Strong enough to give an Admiral some trouble, Robin got 500M Boost as Priority Target for WG, yet She didn't hit 1B ... etc
WG aren't gonna wait till Mihawk hit 1B Bounty to Recruit him, either He was Recruited when his Bounty was still Lower than 500M or He slightly surpassed it, that's all
Shanks hit 1B because He is a Captain of Powerful Crew & probably for Fighting Mihawk who was already a Warlord back then.