The reason that duvet covers are so expensive is that the fabric for duvet covers are more expensive than other fabrics.
So since the duvet cover fabric is more expensive then they must charge more money for the duvet covers.
The higher cost of the fabric for duvet covers is because of the higher fabric costs that are related to quality, easy-care and wrinkle resistant home bedding fabrics.
Duvet Covers range in cost of between $85.00 to as high as $500.00 depending on the quality of the duvet cover and size.
You don't have to spend a whole lot of money to buy a good quality duvet cover as long as you don't get a really fancy one.