Why does it cost almost the same to change the whole screen or just the front glass only ?
Above on the picture you can see the Nexus 5 with just the front glass broken. This phone even if has the glass broken, the screen is still fully functional. Whenever you have the LCD screen broken you might see colour lines from the top to the bottom, colour distortions or you cannot see anything. Some phone screens have the touch functionality also built into the LCD so no responsiveness may stand for the broken LCD screen too. Another group of phones have the touch digitiser together with the front glass so once you get the crack you cannot use your phone at all.
LG G3, G2 – LCD separate, digitiser and front glass as one part, well assembled together!
The front glass in the top mobile phones is currently made of the Corning Gorilla Glass advertised as the strongest glass material available. You already know it is not that strong.