• ITX case: About 40$ for a basic one.

• A decent PSU: About 40$ for a reasonable one.

• ADT-Link R43SG-TB3: 120$ on a good day.

• TB3 cable: ~20$ for a cheap passive one.

That's 220$.

Sonnet Breakaway Box 750 - 300$ on Amazon. You're paying 80$ for a complete product, a warranty and also gain the ability to charge your laptop.

Of course, if you don't want to go the Thunderbolt route, you can do this cheaper via m.2, but then you can't dock/undock nearly as easily.