I built my own CPU cluster with mATX AM3s connected via a switch, powerful processor/RAM and everything else was as cheap as possible. At the time, it easily saved money compared to AWS.
The biggest issue I had was overheating. The small mATX cases don't fit fans sufficient to cool powerful CPUs running at 100% 24/7. So, you may have to get midsize cases, or leave the cases open with fans sticking out, which is louder.
The biggest issue I had was overheating. The small mATX cases don't fit fans sufficient to cool powerful CPUs running at 100% 24/7. So, you may have to get midsize cases, or leave the cases open with fans sticking out, which is louder.