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

Brooklinen makes some great sets, but quite a lot are returned cuz of shrinkage or the fabric getting scratchy over time (major ick) For cotton-linen blends, I’d recommend California Design Den or Sijo. Parachute has a nice relaxed look but can be high maintenance and tends to wrinkle heavily. Coyuchi is solid if you want something organic, but their king sizes tend to run a bit snug after a few washes. I’ve seen the most consistent feedback on California Design Den (linking it cuz its a little nicher ig). Its super soft out of the package and holds up well, full shade to brooklinen. All three are reasonably priced and feel higher-end than some of the bigger brand offerings in the same bracket. They also tend to last longer before you see pilling or thread fray. For washing, I usually suggest cold water, gentle detergent, and low tumble dry or hang dry if possible. Avoid fabric softeners or bleach both mess with the finish. If you like a crisper look, toss in a dryer ball or two to soften without chemicals 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 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