Go to https://cloud.ibm.com/vpc-ext/provision/vs and choose North America > Washington DC > Washington DC 2. Click "Change image" and choose "IBM Z, LinuxONE", then pick Ubuntu. I get $0.09/hour for 1 vCPU/4GB RAM with storage extra.

For fun, then try picking z/OS instead of Linux and see what that software license costs ($/hour). Take a guess first how much you think it'll be and then see if you're right.