Look at Zoro's and Sanji's bounty after they defeated King and Queen. Their bounty is 279-288 million less than King and Queen which shows how inflated their bounty was.
So if someone defeated Jack, their bounty will be around 715 million taking away this inflation. Now I see why Oda put him with Perospero. Both were defeated by a similar character and both could overpower King's Mink without their sulong.
Jack is Perospero overall level. No haki, no mastery over his hybrid form, no named attacks and the reason Tobbi Roppo wanted to aim for an All Star spot.. Jack was not worthy, he indeed is tough, but not on the level of Cracker.