The duvet itself or a duvet cover? For the duvet itself, I've only ever spent a modest amount on them, I spend more on the covers. Personally, I'm not one who sleeps under duvets anyway; I usually take them off my bed before I go to sleep. I do love them for decorative purposes and have generally used the same trick that they use in design spaces which show off bedding setups: I buy two moderately-priced ones and double stuff my duvet cover. Makes it look super plush and luxurious. Super soft to lay on. I don't sleep under them anyway so I care more about the visual element. More on reddit.com

Dark gray cotton fill comforters are definitely going to be difficult to find. Comforters consist of a shell and fill. So when you search, it may have a 100% cotton shell but a polyester fill. 100% cotton fill comforters are not common and not really recommended because while they are breathable cotton absorbs moisture and can get nasty really easily from sweat. Additionally, finding colors for this type of comforter with such a unique fill is going to be difficult. The easy way out is definitely to buy the 100% cotton duvet and use a dark gray duvet cover. If you’re open to other fills, you should be able to do a comforter with the color you want with the cotton shell. More on reddit.com