It has less to do with the series of watch and more to do with the technology involved.
The Apple Watch screen is very tiny. Miniaturization costs money. It uses an OLED display, which is more expensive than other types of displays. The digitizer assembly is laminated to the screen, and the digitizer itself is built into curved glass which costs more money to manufacture.
A good quality replacement Apple Watch 3 screen should cost you around $175 or so.