Really depends on the details of your goals, but why not just VMs?
The "problem" with Pi-like devices is that they're usually not very "normal". The process of provisioning is different, IIRC they only "recently" supported booting off something other than the SD Card, and in the case of the Zeros, you'll either be using Wifi or an external USB Ethernet dongle (over USB OTG no less). Sometimes they need specially compiled version of linux, so you're stuck far from mainline (this was a big component of the RPis success) This may be distracting from your goals of learning about clustering.
I suspect the $10 Pi Zero is about as cheap as you'll get though, depending on your personal costs of case + ethernet dongle + USB power supply, etc.
The "problem" with Pi-like devices is that they're usually not very "normal". The process of provisioning is different, IIRC they only "recently" supported booting off something other than the SD Card, and in the case of the Zeros, you'll either be using Wifi or an external USB Ethernet dongle (over USB OTG no less). Sometimes they need specially compiled version of linux, so you're stuck far from mainline (this was a big component of the RPis success) This may be distracting from your goals of learning about clustering.
I suspect the $10 Pi Zero is about as cheap as you'll get though, depending on your personal costs of case + ethernet dongle + USB power supply, etc.