So, I forget who said it, but Sir Crocodile's bounty would be at least 2x what it was if they knew he was behind Baroque Works/His goals to control Alabasta. Considering Doflamingo is pretty much a successful Sir Crocodile with the added perks of being the don of the Underworld, I think it's fair to give him minimum a 3x bounty increase. This also correlates with Luffy/Law getting 500m a piece after sharing the victory over Doffy.
So that alone puts Doffy at ~ 1,020,000,000
Oda did admit that each Yonko Officer gets an inflated bounty due to their association with the Yonko. I'm sure this also scales with the officers inherent threat levels too. I wouldn't expect Ulti and King to share the same blanket increase from being under Kaido, after all. So add on maybe another 250m for joining Kaido's crew and likely becoming a 4th Calamity.
So I'd say Doffy with what he's done + his threat level, ~1,270,000,000 seems fair.
IDK if breaking out of Impel Down itself would net Doffy extra bounty points. It's possible. Maybe an added 100m to his pot, putting him at ~1,370,000,000.
We're slowly getting there. Slowly.