So obviously it depends on goals; I absolutely agree that up to a surprisingly high point, the best way to actually get the most compute is to go buy the most powerful single machine that will do what you want off eBay or whatever. My goal is the opposite; I want to specifically get my hands dirty figuring out how to actually provision and manage and operate a large number of hosts, even if they're so low resource as to be effectively worthless individually and are still weak clustered. To that end specifically, digikey claims to be willing to sell me a rather large number of pi zero 2w at $15/each - https://www.digikey.com/en/products/detail/raspberry-pi/SC11... - and even cheap used boxes on eBay seem to start at double that. Obviously you need a little bit more to actually run them, but I believe the only thing you actually need is a USB cable and then a single host computer can provide power and boot them over usb.