For the cost of a $35 Raspberry Pi, you can rent a considerably more powerful "small" EC2 spot instance for 7 months, or a more comparable "micro" for 16 months. Much longer if you don't need it running 24/7, of course.

Granted, that's no substitute if you need something with physical I/O ports, but if you want to learn how to set up a real compute cluster, EC2 is a more realistic environment.