1.You don't have to pay anything for maintenance of certification(unless it is expired).
2.You can take Platform Developer II exam if your Platform Developer I certification is not expired and you have completed all the prerequisites.
3.If your Platform Developer I certification expires and you also hold the Platform Developer II certification, the Platform Developer II certification will be placed in a suspended state. You will then have one release cycle from the expiration date to re-certify as a Platform Developer I, at which point the suspended Platform Developer II certification will be reinstated.
For more information about certification expiration read this: https://trailhead.salesforce.com/help?article=Information-about-Certification-Expiration