What makes Thunderbolt 2 adapter more expensive than TB 3 or USB-C? I have the 2015 MacBook Pro and I was searching online for a hub because I need an Ethernet port and more USB ports.  

My first thought was "Oh, no problem. I'll just get one for 5 or 6 dollars".  So, I go online and check what it costs. The average was around 100$ up to 300$ or even more, at the same time, you can buy an identical hub for the Thunderbolt 3(USB-C) connector with price under 15 bucks.  

How is this even possible? Are Thunderbolt 2 and 3 that different? 

I will get a typical USB hub, it's not a problem, but I would like to know Why does this huge difference exist?
Thunderbolt is expensive because of two reasons:


Licensing fees required by Intel (initially co-developed with Apple but Intel gained all rights in 2012)
Technical requirement of having a Thunderbolt controller module on both the host and peripheral device


Because of the licensing requirements that Intel had, any peripheral product had to pay a royalty to Intel.  So, it was expensive to PC and motherboard manufactures because they had to pay both a license fee and incurred development costs to integrate it into their products.

In comparison to USB, the licensing fees are nominal, making it very cost effective to utilize USB in their products.

However, with the introduction of Thunderbolt 3, Intel removed the licensing fee and they started integrating the technology directly into their processors meaning that it’s now much easier and far cheaper to integrate Thunderbolt into new computers.

As far as compatibility, Thunderbolt 3 is backward compatible with both 1 and 2.  However, it will require an adapter because unlike Thunderbolt 3, Thunderbolt 1 &amp; 2 require an active (powered) cable; the protocol is still the same.


  you can buy an identical hub for the thunderbolt 3(USB-c) connector with price under 15 bucks.


This is a common misconception.  Thunderbolt 3 only uses the USB-C connector, but they are not the same. Thunderbolt 3 is several technologies in one as opposed to just USB:


PCIe
DisplayPort
USB 3.2 (soon to be USB 4)
Power Delivery


It’s the PCIe bus that is key here.  It allows for greater flexibility in terms of peripheral interconnect.  For example, you can hang a PCIe expansion chassis of your laptop and add external GPUs for gaming or data/number crunching applications.  

From a personal standpoint, I have used both USB and Thunderbolt Ethernet adapters and found that the Thunderbolt adapters utilized less CPU, especially during heavy load, than their USB counterparts.