If Mihawk doesn't have a comparable bounty ti Shanks then it Just shows that he is not his level.
Luffy despite having a crew, a fleet and other things his bounty isn't higher than Kidd and Law who don't have fleet or territories.
So don't find any excuse, Shanks's bounty isn't boosted by crew, territories or other things, if he has way higher bounty than Mihawk It Just means what It means.