I researched and found Monkey D. Dragon to be worth 850,000,000. But what about Roger?
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.