I have a 22 oz down quilt that costs $240. It is warm below freezing for me. If that's cheap to you, that's great. If it's not, good luck...

It is cheap for ME because I use it all the time - just used it on the coast, will use it again next weekend on a camping trip, will use it another 12+ nights backpacking in July and August, as well as a number of weekend overnights in May and April. Figuring how much it increase my comfort on the trail plus my comfort each night I use it, I figure it is the cheapest item in the pack. Anything that keeps me in the backcountry comfortably is worth that $240.

The other qualifier is that all things are relative. I much prefer spending the money one time for a light, warm down quilt and sleep comfortably for years than spending $100 on something less warm, bulkier, and heavier that I won't be so comfortable in. I won't be packing extra clothing items to make up for the lack of warmth, and won't be uncomfortable either while on the trail packing it along (no unnecessary pounds in the pack) or at night (no need for extra layers while sleeping). I know that with proper care my quilt will last me "forever" (likely til the end of my backpacking career). YOU may find that your budget just won't stretch to fund a high quality bag. I have a lot of other things I use the quilt for. It fits in the Search and Rescue pack and it also works with my hammock, as well as opening flat to work as an extra layer on the bed - as it did when the heater died on the coldest week of the year.

Your personal equation may be different - you may have different priorities.

Spending $500 on a bag that IS WARM to the rating stated is not only lighter than a cheaper bag, warmer than a cheaper bag, and comfier - it can mean not having to spend more money on other things to make the cheap bag work, and not having to replace it in a few years (synthetics wear out pretty quick with frequent use; the more times you compress and re-stuff and re-pack one the more quickly the fibers break down resulting in a not-warm-enough bag).

Spending the extra dollars on a high quality down item, rather than the low quality gear (800 fp is high quality, 650 fp has lots of feathers and consequently will have more weight and bulk than an equivalent bag with 800 fp down) is worth it to me as well. But it may not be the same for someone who is out two or three nights, every once in a while.