Bed, Bath & Beyond plus Kohls have killer deals on nice ones. I'm a big fan of either clean crisp white sheets for the bed, (around 500 thread count)

or Sheex brand of sheets. I've got them in Black, Grey and like a violet color. They do a good job of wicking sweat in the warmer months. They're a little pricier but they're pretty awesome and hold up so well to wear and tear.

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I make my own because no commercially available one was just right. Its fun and easy and a great way to get started on a sewing machine. All straight seams, maybe curved corners, and a few tie-ups. I get used flat sheets at the thrift shop, cheap and you can feel the thread. More on reddit.com

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

I see no problem with that strategy, honestly. The duvet cover often is the most expensive element in many instances. For example, I always recommend a linen canvas duvet cover since it's so damn durable and breathable out of all the fabric options available but it's insanely expensive ($500+ retail). This usually leaves little budget left over for a duvet insert for most people -- even when they find great deals at a few hundred dollars less from places like Ebay --- so it is not a new concept. I tend to also highly recommend silk inserts since they're great for hot sleepers and they are pretty reliable all-around (cheap ones to expensive ones). Hell, I have a cheap all-season silk insert from Costco (SilkHaven brand) that works really well for the price I paid for it, so buying budget-friendly duvet inserts sometimes is not a bad thing at all! The more expensive silk inserts tend to be either really nice outershell options or really high-end silk grades (6A) which raises the price quite a bit (but doesn't increase the breathability of the silk fill too much, overall). Overall, though, you'll probably do pretty well with a more average-priced insert as long as the filling material you're buying is breathable and decently durable. More on reddit.com