if I try CHAPTER 3.18 Lab – Network Mess on AWS free tier

As a Solutions Architect Professional you should be able to evaluate the CloudFormation template provided to determine the expected costs. Seeing as there are EC2 instances included in these templates there will be some bill if you execute them in your AWS account. 

That being said:

• The parameters declare the EC2 instance type as micro so the cost will be minimal. Especially if you clean up after yourself like you should.

• The costs would be covered under the free tier assuming your AWS account is new enough. 

https://github.com/ACloudGuru-Resources/Course_Certified_Solutions_Architect_Professional/tree/master/lab-networking

https://calculator.aws/#/