When you buy top quality cycling shorts you are paying for greater comfort, better fit, higher performance and durability.
This performance is achieved with key features of better bike shorts, including the choices of fabric and chamois, the number of panels and the care taken in orienting the fabric weave in each, the quality of the seams, and materials used in details such as grippers and hems.
Sure, you can find garments sold as "bike shorts" for as little as $20. The problem is they are likely to fall apart after a very few rides and washes, and are unlikely to provide much in the way of comfort or performance at all.
Cheaper bike shorts are usually made with basic nylon or polyester. Adding fabric properties that make the shorts feel nicer against the skin, more comfortable, durable, breathable, wicking, and sun protecting, all add considerably to the cost.
Premium chamois are made with foam technology that does a better job of cushioning, transporting moisture, and controlling bacteria. A premium chamois will be more comfortable and more durable.
If you have a real interest in cycling, you'll find a premium short will give you greater comfort, and can be expected to hold up to regular rides and laundry for several years. In fact, you may find cheap bike shorts turn out to come at a surprisingly high cost over time.