In chapter 957, it is said that no pirate has ever surpassed the bounties of Roger and Newgate, and since Xebec is a pirate, his bounty must be less than WB's bounty (5,046,000,000), the bounties of the pirates who are at the top of piracy, like the Yonko and Roger, have a Japanese wordplay called "Goroawase" where turn words into numbers, since we don't have much information about Xebec I will use his name for the wordplay.

His Romanized name is "Rokkusu Dī Jībekku" and his name in hiragana is "ろっくす・D・じーべっく" so doing the Goroawase with Rocks we have

and with Xebec we have

Using only the largest numbers for his bounty, with Rocks we have the bounty of 4,X62,940,000 and with Xebec we have the bounty of 4,X48,290,000.

My final guesses are

I changed the X to 9 because Linlin's bounty is about 300 million greater than Shanks', Kaido's bounty is about 300 million greater than Linlin's, so Xebec would have a bounty about 300 million greater than Kaido's

The another guess

I changed the X to 0 because it gets a little smaller than Shanks' and that would show that the current pirates are just as strong/dangerous as the ones from the past era and also because I like the theory of Shanks being Xebec's son (even though the chances of that happening are tiny) so father and son having close bounties would be interesting