[lvillani]

Most modern smartphones have the LCD, digitizer and glass all fused together. It improves image quality and reduces the distance between the glass and the image. This, however, greatly increases the replacement cost since you would then have to replace the entire unit when you break it. Also, higher resolution screens are more expensive.

I think the 3GS is old enough to have these layers as discrete components that you can replace one by one, hence the lower cost.

[verall]

yes, the 3gs didn't have loca, although the 4 and beyond did. Android phones tend to have much more expensive LCD's which since about 2013 are all loca'd to the glass and digi, so when the screen breaks on your s7 it's a very pricey fix.

At the shop I worked in, in the case of broken glass but fully working LCD, on a galaxy s3-s5, we would spend about an hour holding the screen assembly under a heat gun and slowly peel the glass off the LCD with a playing card. The we could put on new loca and glue a new piece of glass on, which cost ~$10. We charged like $120 for the because it took forever and there was like a 1/4 chance you broke the LCD and then replacing the assembly was like $110.