Well, he also needed to add a cooler and a bunch of tubes & eight mounts to get coolant running on each Pi, eight ethernet cables, eight power cables, a power supply with enough outputs, a 16-port ethernet switch, and (correct me if I'm wrong) eight SD cards, if only to store a bootloader that can do pxe.

So eight Pies alone do not make a complete cluster, just as one Ryzen alone does not make a complete computer. A few times eight times cheap can turn out to be quite expensive, depending on how much cheap actually is.

I haven't paid much attention to component prices recently but my rule of thumb for budget builds is to start with $80 for each component that isn't a GPU or CPU. Go with stock cooler, grab 80 for PSU, 80 for mobo, 80 for RAM, 80 for storage, see where you end up. In the same ballpark, but the real computer will pack a whole lot more punch (even if you had less RAM total).