Wind Hard Tiny Pro 10D 800FP Down Sleeping Bag Quilt

I was able to push a wind hart tiny pro quilt from AliExpress down to -6 Celsius when wearing all of my layers. Might be a good option if you are open to using a quilt

In that price range I’d go with a quilt from aegis max or ice flame. Not as high quality as more expensive options but i think will offer the best bang for your buck in terms of down. I have used a kelty cosmic 20 car camping and it was not even warm to 30 degrees F (-1 C). It’s also quite heavy and bulky.

I actually quite like my aegismax. Decent enough, it frequently goes inside a 20° quilt to buy me comfort down to 0°F

I have these insanely tiny down sleeping bags from Ali Express. I typically use them as a second bag when cold weather camping but have used them when staying at airbnb etc. 

I personally use a wide 20F-ish comfort quilt with an Aegismax Mini zipped up inside of it. It's mildly fiddly, which is slightly annoying when it's "exposed hands stop working pretty fast" levels of cold. But it works. I estimate that this setup would be okay down to about 0F. By that, though, I mean single-digit expected temps that could drop down and hit 0F. If there were a risk of even colder temps, I would want a genuinely high-quality, no-BS mummy bag. The risks of compressed-down cold spots and drafts are not something I want to screw around with when it's super-duper-deadly cold.