Eyewear is really not much different than any other commodity. If it cost more for the consumer, it's usually because there's more technology, labor involved and money tied up in raw materials. The result is typically a higher quality product that will last longer and work better.
Inexpensive plastic frames are typically injection molded. That means that petroleum based melted plastic is squirted into a mold and out pops frames. Usually the colors are monochromatic and lifeless. Inexpensive metal frames are made from low quality pot metal associated with skin allergies and not staying in adjustment. These are all produced in China.
High quality plastic frames are made from plant based acetate, most commonly produced by Mazzucchelli in Italy. They are cut from sheets, added to and bent into shape by hand. It takes as many as 200 steps to make a fine acetate frame.
The best metal frames are made from high quality, hypoallergenic materials, typically surgical steel, stainless steel or titanium, also largely produced by hand.
The results are timeless frames that can be relevant and functional for years rather than disposable.
Glasses are no different than clothes, cars, purses, jewelry, food, basically anything you can think of where paying more results in getting more.