A bounty is a resume. Mihawk's bounty being frozen for 20+ years, and him having 0 crew or territory, means it won't surpass the 2 billion mark. And I say 2 billion, simply because (1) it has to be higher than Luffy's (2) it has to be lower than Blackbeard's, since Blackbeard has a crew and territory.
I think Oda might not even show Mihawk's frozen bounty, because it has to be in proportion to the other pre-time skip Warlords, and that would dehype him. Bounties are just hype tools after all.
Moriah fought toe to toe with Kaido and his frozen bounty was 320 million. Doflamingo was the most powerful in terms of influence and he was 340 million. There is no chance in fuck, his frozen bounty is like 2+ billion. That's stupid. It's probably 3x Moriah's and Doffy's bounty. Considering BM/Kaido's bounties are 3x their first mates. So I'd say around the 1 billion berry mark, but I could see Mihawk's frozen bounty being as low as 600-700 million.