Depends on what we're factoring in. If we were to take literally everything into consideration, those things being:
- Knowledge on Oden's journal
- Daughter of Kaido
- Potential to guide the strawhats concerning Laughtale
- Her general strength
- Mythical Zoan
- Being an Oni, which for all we know could be an important race
Then her rightful bounty should be as high as 800 Million - 900 Million, you could even argue 1 Billion but that is ludicrously high. Honestly anything over 100 Million is pretty insane for a first bounty in general but I think a realistic number for Yamato's first bounty should be anywhere from 400 Million to 600 million, and I'm thinking it should be on the lower end of that so below Jinbe's post wano bounty but higher than every other strawhat's.
Kind reminder that bounty =/= strength so its fine for Yamato to have a lower bounty than characters she's stronger than.