I think the repair cost of these devices needs to be put into perspective. We aren't just carrying around a mobile phone when we have an iPhone 6 or a Samsung Galaxy S6 in our pockets and purses. They've come a long way since the old Nokia 3310! We have with us an extremely powerful computer using some of the most sophisticated and contemporary technologies known to modern man.
They can be broken down into a few basic components or set of components:
1. The most important being the motherboard, which is the brain and nervous system of the device;
2. Then of course the screen assembly, consisting of the glass lens, the touch screen (digitizer) and LCD (display). This acts as the interface which allows us to interact with the programs on the device, and with the rest of the world through the internet;
3. Lastly, the housing and all the peripherals of small parts, including cameras, charge ports, microphones, speakers, battery and buttons etc.
When seen this way, it can help to understand why it can cost from $170 upwards to replace a screen assembly on a device which retails for $800-$1100. It's replacing a major component on a very expensive and sophisticated piece of equipment.
The same holds true for laptops and tablets (iPads etc).