You've got to have a decently made chassis to accommodate and cool a full-sized card and a meaty power supply for the latest and greatest GPUs. And that's before you add any docking functionality for extra ports.
The cheapest thing I've seen to date is the Alienware Amp which sells for around $150 but uses Alienware's proprietary port, which IIRC, is a bit slower than TB3.
I haven't seen them tested side by side but actually both TB3 and the Alienware Amplifier offer 4 lanes of PCIe gen3. So they should be pretty much identical.
As for the lower-end pricing, it's very feasible to fit a laptop-oriented MXM GPU moodule in a lower power envelope. The GTX 1060 mobile variant only draws 80 watts, and the GTX 1050 mobile sips power at 50 watts. They do obviously need active cooling, but not a LOT of active cooling.
Note that the 10-series of GPUs are not cut-down versions like previous generations. The 1050ti mobile is just as fast as the 1050ti desktop card at same clocks, they bin the chips so it can work with less power.
It's very reasonable to imagine a sealed laptop dock (as opposed to a full enclosure) with a 50 watt 1050ti inside selling for $300. Comin' soon!
You've got to have a decently made chassis to accommodate and cool a full-sized card and a meaty power supply for the latest and greatest GPUs. And that's before you add any docking functionality for extra ports.
The cheapest thing I've seen to date is the Alienware Amp which sells for around $150 but uses Alienware's proprietary port, which IIRC, is a bit slower than TB3.