Fast lenses are usually genuinely more expensive to make, not just priced higher for marketing. A lower f-number at the same focal length means a larger entrance pupil, so the lens needs larger glass elements. Those larger elements are harder to manufacture, heavier, and more expensive.

The bigger challenge is optical design: when light enters through a wider opening, it is much harder to focus all those rays sharply onto the sensor while controlling aberrations and distortion. That often requires more elements, more complex designs, and tighter manufacturing tolerances.

Those added optics increase size and weight, which can also require stronger focusing mechanisms, sturdier mounts, and better overall construction. In many cases, fast lenses are also aimed at professional users, so manufacturers often pair the optics with higher-grade build quality, which raises cost further.

So the price difference is usually driven by real engineering and manufacturing difficulty, plus the fact that these lenses are often built to a higher standard for a smaller, more demanding market.