There are a few reasons why vintage mid-century modern furniture pieces may be expensive. One reason is that they are often considered to be highly collectible, and as a result, demand for them can be high, driving up the price. Additionally, vintage mid-century modern furniture is often of high quality and was well-made, using materials that were built to last. This means that these pieces can be in good condition even after many years, which can make them highly sought after by collectors and others who appreciate the style. Finally, the limited availability of these pieces can also contribute to their high price, as there are only a certain number of vintage mid-century modern furniture pieces in existence.