You seem to be under the misapprehension that all lenses should cost what a 50mm f/1.8 costs. The 50mm lens is actually the outlier. The focal length lends itself to simpler designs.
An 85mm lens, to achieve f/1.8 must have glass that covers an aperture opening of 85mm/1.8 => 47.2mm vs. a 50mm/1.8 => 27.8mm. So it requires bigger glass elements throughout, to begin with. It will need additional help with chromatic aberration and distortion correction for the longer focal lengths, so additional elements.
50mm f/1.8 diagram:
85mm f/1.8 block diagram:
The cost isn't arbitrarily higher. It has more materials and design going into it.
In terms of non-obvious features, 85mm is not only going to give you a tighter framing and a narrower field of view and more "reach", but also a shallower depth of field, and possibly better corner performance on full frame. And on full frame it's an amazingly good fit for portrait photography. Less so on a crop, where it's a little too long for framing anything other than headshots.