DigitalOcean reserved IPs are publicly-accessible static IP addresses that you can assign to Droplets and instantly remap between other Droplets in the same datacenter. Implement a failover mechanism with reserved IPs to build a high availability infrastructure.
Reserved IPs are free when assigned to a Droplet.
Reserved IPs cost $5.00 per month ($0.00744 per hour) when reserved but not assigned to a Droplet, due to the shortage of available IPv4 addresses. You are not billed unless you accrue $1 or more per reserved IP.