Level 6 seems to use special requirements. It is unknown if level 1 has a minimal value, although Buggy commented that every prisoner from level 2 that escaped with him holds a bounty higher than his 15,000,000. Levels 3 and 5 require bounties of 50,000,000 and 100,000,000 respectively, and the values required for others levels were not revealed. While several prisoners from Impel Down do not hold any specific value, Impel Down orders its prisoners from the bounty on their head. Newcomer Blackbeard is an exception, having managed to get in by capturing Portgas D. Most of the Seven Warlords of the Sea had former bounties since the first requirement to join the group is being recognized world-wide so one's notoriety scares criminals. Gecko Moria was first introduced by Robin as merely holding a bounty higher than Luffy's. When Igaram first mentioned Crocodile's former bounty, he said it was 80,000,000 while it actually is 81,000,000. For example, Nico Robin's specific bounty was not made clear until after it was known she had one.
In the series, many bounties are revealed - either by the author in a reference box, said by a character (like Shakuyaku naming the nine Supernovas not part of the Straw Hat Pirates), or a bounty poster being revealed (although the value may not always be fully accurate or fully shown, as with the Red Arrows Pirates).