No referral links here...
| Item | Quantity | Link | Price |
|---|---|---|---|
| Atomic Pis (worker nodes) | 4 | amazon | $35x4 |
| Raspberry Pi 4 (master node) | 1 | amazon | $42 |
| Dupont Wires - remember you need four wires to carry the current (2 positive, 2 neutral) | 1 | amazon | $7 |
| 200W - 5v, 40amp Power supply | 1 | amazon | $17 |
| Total | ~$210 |
You'll need ethernet wires and a switch to handle the networking. I had those laying around and don't have any special recommendations.
As for the chassis, I'm currently using legos but have seen people do interesting things with standoffs.
Again on power, a dupont wire isn't enough to carry 5v by itself. You'll want to use two wires for each side (4 in total).
The Raspberry pi for the master node isn't really important. I did this because I had a RPI laying around. You could use an Atomic Pi instead.