You can find incredibly cheap (nearly free) used servers for pickup if you’re patient. If you have cheap electricity (or better yet, provided with your lease), a 2013-era dual Xeon server is pretty compelling at $50.
Apple didn’t make that many, so they would be tough to find. And they were a really pain to actually use. I didn’t find them very friendly at all to work with physically. Very polished, but had a feel of being over engineered.