My predictions
Zoro 450 000 000 - 500 000 000
1. Soon Jinbei will join, and he has a bounty of 400 000 000+, and I think Zoro should still remain a second biggest bounty in the crew.
2. Zoro's new bounty has always been bigger than Luffys ex-bounty. So when Luffy went from 30m to 100m Zoro got 60m, when Luffy went from 100m-300m Zoro got 120m. So I think Zoro's new bounty should be 400 000 000+.
3. Zoro could easily one shot guys like Caribou (210 000 000) or pre-time skip Luffy (400 000 000) and Kid (315 000 000) etc.
4. Zoro has also one shoted a Pacifista, and they are stronger than any supernova was.
5. I think curent Zoro is on pair (slightly stronger or slightly weaker) with characters like Jinbei (400 000 000+), Law (Formerly 440 000 000+), Ace (550 000 000 ) Enel ( who would have 500 000 000 if he was a pirate), Don Chin Jao (500 000 000+ in prime) etc.
Luffy 600 000 000 - 700 000 000
Clearly Luffy should have a bounty higher than Zoro. If bounties were all about the strength, then I would say Luffys bounty would be between 550-600m.
But considering everything he has done in the past, considering he's the captain of the crew, a son of Dragon, Grandson of Garp, Brother of Ace, disciple of Rayleigh etc. I think 650-700m is more appropriate
Also I think his new opponent will be Doflamingo, whose new bounty will be, IMO, 600-700m.
Sanji 400 000 000 - 450 000 000
Not much to say here, he's a little bit weaker than Zoro, so a 400-450m bounty seems right.
General Franky 250 000 000 - 300 000 000
In the movie Z, we saw Franky destroying several Pacifistas in 10-15 seconds
I think that Robin is on pair with 100-200m bounty pirates, but she will probably be a key character in revealing what happened in a void century, which will eventually lead to the destruction of the world government. And closer she is to the end of the grand line, the more dangerous she becomes. so if she gets 200+m bounty it will be because of that, not her strength.
Brook 150 000 000
Usopp: 100,000,000
Nami: 50,000,000
Chopper 45