I've been shopping around for gears, pulleys and other transmission components for building a milling machine over the past few weeks. Many of these parts are so rudimentary and ubiquitous, yet the prices are very high. A ribbed pulley, for instance, about 3 inches in diameter often sells for over $30 online. Many of these parts are either stamped or cast, making them cheap to manufacture. What factors into the cost of these components?
In such cases you don't pay for the rubber, but for the administrative and storing costs. Manufacture a such rubber and then try to sell it on the ebay. With taxes, all of the law regulations, with post costs, etc.
For example, if you only want a rare screw to repair a device, you can easily pay even tens of dollars for that. Why? Producing a kilo of such screws would only cost fewer as a single dollar. But storing a thousands of similar boxes full with rare screws, and then sent them on post to countries thousands of kilometers away, it costs much more.
And, there is another effect, which an already deleted answer said. They won't calculate the price on the costs plus a profit, but they will see how is it priced on the ebay.
It is a highly unfortunate feature of the global society.
You can make significantly cheaper your costs if you pay a lot of them, and know the market (so you can buy it from cheaper manufacturers), and pay from them directly. But you can only do that if you aren't a hobbyist, but also a large manufacturer.