I researched and found Monkey D. Dragon to be worth 850,000,000. But what about Roger?
In chapter 957, titled "Ultimate", it was revealed that Gol D. Roger's bounty was 5,564,800,000 Berry, the highest pirate bounties in history.
The partial bounty poster was also shown, although it didn't show the bounty amount.
The answer to that question, as far as I know, hasn't been revealed yet. His bounty from when he was alive is unknown for now.
For that matter, Dragon's bounty being 850,000,000 is a false rumor, as no official sources have revealed his bounty either. Besides, a bounty this comparatively "low" would contradict Dragon's status as the most wanted man in the world, as Jack and Charlotte Katakuri both have bounties that equal or exceed a billion.