How much does an Elastic IP address cost per month?

AWS
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One of the cheapiest AWS regions is «US East (N. Virginia)».
An Elastic IP address costs $0.005 per hour there, so the monthly cost ≈ 0.005 × 24 × 30 = $3.6.

aws.amazon.com/ec2/pricing/on-demand#Elastic_IP_Addresses

2

See also: How to preserve an AWS EC2 instance's public IP address after it is stopped and started again?