alt Hacker NewsReally 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.
I recently built a Plan 9 cluster of 8 raspberry pi zero (2w). From my supplier (digikey) the 2W was the same price and has 4X the cores!
I think it looks quite cool: https://x.com/andreer/status/2007509694374691230
Instead of having to use lots of dongles and usb ethernet, I just wired them all up using brass rods, a small 5V power supply in the base, and boot them over WiFi (just the kernel and wifi config on the sd card).
Raspberry pi's are cheap, easily available, and there is an absolutely massive trove of information about them on the internet. And the scale means that the linux implementation is very stable and "just works" to a degree that is extremely hard for other SBCs to match.
Sure, VMs are the logical choice, but not everything has to be logical. Real hardware does feel more real :-)