Oda usually uses word plays for bounties and after given Queen's bounty a bit of thought, and one possible pattern for it is 1.
32 = (San-ji) so assuming this is true, then I thought maybe King's bounty will be based off Zoro, and a possibility here is "green" which is "midori" in Japanese. A bounty incorporating
Mi-do-ri (3-10) could be 1bil & 310mil so 1.31bil
.
Considering the dynamic btw Queen and King is similar to Sanji and Zoro, it's a possibility ^^ and if this is indeed the case, it'd kinda make sense why Oda has hidden till now