If what Bao said was true, Marco has a quite low bounty for someone with a mythical fruit and serving under WB for at least 30y, much of this time as his strongest crewmate.
I always thinked that Marco would be around 2b, and that Teach bounty jump to more than 2b was because he defeated Marco, with Teach getting something around 1b for defeating Ace(550m), leving the Shichibukai, Invading Impel Down and taking WB fruit and them another 1b for ending the WB pirates and finally becoming a Emperor, but seems that thats not the case.