I have been working in some similar project the last year. My client was ( ) , we deploy one Proxmox Server ( it's a virtualization platform ) on Amazon EC2 medium instance. We deployed our own template and create the instances, the cost per month on AWS was 105$ and we hosted 25 instances without problems.
Anyway I could create a new template , create the accounts for the user and associate it to your bill account using IAM and permit to your accound user deploy , start and stop all the instances of the other users account , that's permit to reduce the total cost of your labs.