I've been looking as well for a lightweight rear rack that doesn't brake the bank. I haven't found any carbon or ultralight racks being sold on Aliexpress.
I gave up and decided to modify an existing inexpensive aluminum rack. (Purchased a $12 USD one from Aliexpress) I took an aluminum rack and lightened it up some by milling out some of the center section. I have the rack down to about 8oz. Then I made/machined my own side supports out of Aluminum and was able to get each support down to about 3 oz each. Total weight is just under a pound.
I plan to cut off the center and front side rail supports
The most expensive part to date was purchasing an "Old Man Mountain" thru axle with rack eyelets on it. this added a couple more ounces to the bike. (The price is silly for such a simple design, but I didn't want to machine my own version. (Time is money scenario)
Now that I viewed the video of the carbon rack that was posted here, I will start planning to fabricate a similar one.