There will be no costs incurred for creating the AMI itself. The only cost that you might have to pay is for the micro instance. Usually, it is $0.02/hr but may vary with availability region - see https://aws.amazon.com/ec2/pricing/ for details.
EBS volumes get copied once the instance is launched and hence will require you to follow the regular EBS volume and snapshot fee. You can navigate the link above for official AWS guidelines on pricing.
But to answer your question, the cost incurred will be close to negligible as AWS offers most services in their free tier (up till certain limitations)
Hope this helps!