Down and feathers have been used for bedding and clothing for centuries. Geese, ducks and even chickens have been used. Goose down is recognized as a very good natural insulator for bedding, with one exception, white duck down. However, down is very expensive due to limited supply (in many places, down duck is considered an endangered species), so most of our conversation here will focus on white goose down.
Filling power is a method of measuring down quality. The higher the number, the better. Specifically, filling capacity measures the amount of cubic space that an ounce of down can fill. For example, an ounce of 750 fill power drops will fill 750 cubic inches of space, and an ounce of 550 fills 550 cubic inches of space.
What does this mean to you? A 60-ounce 750-filled duvet is lighter, warmer, and fluffy than a 70-ounce or even 80-ounce 550-filled duvet. Therefore, although 550-filled quilts may not be too expensive, if you are looking for a lightweight, warm and fluffy duvet, this may not be a better choice.
Downfill is usually rated from 550, 600, 650, 700, 750, and 800. There are 1,000 fill power supplies available, although the number is very limited, making it more expensive. Good quality duvets are usually filled with 650 to 750, and some use 800 or even 1000.
Asian down can be found in almost all "cheap" or "cheap" duvets, even those that claim to be good duvets. There is a reason for this. Asian down may be very inconsistent in quality and cleanliness, and it is much cheaper.