Luffy: 225,000,000 - 250,000,000. Seems about right. Lower than than the current Schichibukai with the lowest bounty, but still high enough to be considered a serious threat to the World Gov't.
Zoro: 125,000,000 - 140,000,000. Kaku's douriki level is 55% that of Lucchi's, so 125-140 million seems about right.
Sanji: 120,000,000 - 135,000,000. Whatever Zoro's bounty ends up becoming, Sanji's bounty will be about 5,000,000 less. He's still at approximately the same level as Zoro, but Zoro’s bounty is nevertheless a little higher.
Usopp/Sogeking: 110,000,000 - 125,000,000. His bounty is 10,000,000 less than Sanji's. I suspect Usopp will do something that will cement him as one of the top tier Strawhat fighters. Not as a melee fighter however, but as a sniper.
Nami - 35,000,000 - 40,000,000. Based on Calipha's 630 douriki level (about 16% that of Lucchi's douriki level).
Chopper - 45,000,000 - 50,000,000. Based on Kumadori's 810 douriki level.
Robin: 79,000,000 – 400,000,000+. She's the wild card here. Her bounty could be anywhere from 79,000,000 to 400,000,000 or more. Recall that a 'criminal's' bounty isn't really based on his/her ‘power level’ but on his/her threat level to the World Gov’t (nevertheless, there is a direct correlation between a ‘criminal’s’ power level and his/her threat level, which is why many in the OP world use a person’s bounty level to gauge his/her strength). Since Robin, with her knowledge (perceived or otherwise), is the only Strawhat capable of bringing down the WG, I suspect her threat level will become quite high after she's rescued. If her bounty level ends up being as high 400,000,000, then Robin will then need her crewmates more than ever to protect her.
Adding together all the lowest bounty estimates for each crewmate, and assuming Robin’s bounty is quite high (about 340 – 400 million), then the crew’s bounty could top 1 billion beli. Nice. If this happens, people will begin perceiving the Strawhats, not as that crew that's led by a 225,000,000+ beli captain, but as that crew that’s worth 1 billion beli in total. I like this, as it would emphasize that the Strawhats are really all about the team, and not just about one man.