When Apple first released the iPhone, the lcd (that produces the image) and the digitizer (that detects your finger movements), were fused together. That means that if you cracked the digitizer glass, you still needed to replace the whole assembly. When the 3G/3GS came out, Apple computer, in their infinite wisdom, made these two parts separate. The real winner of that decision is the iPhone owner who cracks the outer glass. Because the cost of the part is lower, the cost of that repair is far more reasonable.
Now enter the iPhone 4 in all its glory. The fantastic retina display lcd is fused to the digitizer just like they did with the first generation iPhone. Regardless of why they made this decision, it means the cost of the part to replace the glass on the iPhone 4 is four times the cost of the part for the iPhone 3G.
At first, these parts were amazingly expensive. The costs have gone down enough for us to be able to offer this repair to our clients for a reasonable price.
Back Glass Replacement: $95
Front Glass Replacement: $159