1. Location (and internet) has a lot to do with it. When I lived in Japan, even a "good" slimline could go easily for the equivalent of $100 +. In my "smallsville" USA home town, that same pen would not sell for more than $25.
2. The artistic creation has a lot to do with it too. In the OP the slimlines with designs were attractive. Attractiveness draws people in. The more drawn in, the more sales.
3. Quality. Its "good enough" will not bring great prices. Each and every pen should be made as though it was the only pen - for the slimline to break out of the bottom rung. I know some here make their living by sales numbers. If I were making or supplementing my income with pen sales, then sales numbers would be important. But since I am not, I treat each and every pen as though it were the only one I am making. "Good enough" and "Getting it out the door" cuts it on the lower end but not on the higher end. ON the higher end, it is about "IS it finished? What does the fit and finish look like under a 10X or 20X magnifier?"
4 Apples and Oranges. With number 3 above, there is the argumentative confusion between two different models, both distinct from each other. Slimlines are generally seen as the lower end and priced that way. Then there is the OP link that shows a slimline for nearly $200. The artistic value added with the matching segments top and bottom - push it into a different class of artistry.
Understanding the target audience AND creating/finishing the pen accordingly is more important than classifying a pen into one category.