At most, I think Red Leg Zeff could have a bounty over 100 mil. But I don't see it being that much more than 100 mil. Again, at most.
I mean, 67M doesn't sound like much nowadays when it's all about the billions but then again look at these bounties of established Grandline pirates
Arlong 20* (bribed his way down, probably worth around 50-70)
Mr3 24M
Mr2 32M
Brooke 33M
Sarquiss 36M
Roshio 42M
Franky (44M, first bounty)
Bellamy 55M
--- (Zeff 67M?)
Mr1 75M
Nico Robin 79N
Crocodile 81M
Oda really screwed up when he made friggin Pekoms 330, Tamago 429, Chinjao into a 500+ and Cracker 800+... 300 used to mean something when Kuma, Doflamingo, Moria etc were around that amount. Now you have Jack at 1B when he shouldn't be any stronger than Kuma or Doffy going by Marineford.
Blackbeard was looking for a 100 million + bounty. I guess he could have just beat Coribou (190), Bellamy (195) or Caribou (210) instead of going after Luffy who just beat hyped Schichibukai Crocodile.