It would just be my cost. I actually have an assembled external one with cable and 3d printed case for ~$24, a bit extra for the cable and connectors. I'd need to pick up another ESP board to put together an internal one, but it would be $20. Let me know if you're interested.
Unless prices have dropped, I paid ~ $13 for the assembled boards, and $6.50 for the ESP32s with ordering in bulk (5 boards), so it would likely be more expensive otherwise build your own.
