I am one of the culprits here: I made a top bag (quilt top, single layer bottom with zipper) out of M50. I previously made a similar, but warmer bag out of a thru-hiker kit in M90 for slightly cheaper. Regarding the very high cost of M50, I agree: for my purposes I saved 3 ounces on the shell, and paid quite a premium for it.
For better or worse, I justified this by considering the following:
At this point I save a lot of money by making my own stuff. I came about this project because I wanted a sub-pound three-season bag, and looked at my options. The only similar product I found was through Nunatak, and would have cost me $400+. After selling some extra down and materials I did this for about $165 and still have left over M50 for a balaclava, stuff sacks, etc. Not cheap, but much less expensive than any alternate options for the weight, and most people would consider $165 for a high end UL sleeping bag to be a steal. Also, when it seems appropriate, I do like to support cottage manufacturers and companies/people that seem to be promoting ultralight backpacking and developing/sourcing quality materials.
Admittedly though, I didn't 'need' a bag made from M50, I wanted one, and talked myself into buying the materials. I had some gift money from the holidays, and making an awesome new sleeping bag with exactly the materials I wanted seemed like a pretty cool way to spend that money. I think that most people would be very well served by buying thw OWFINC 0.9 ripstop and 800 fill down (less expensive than the 900 fp from thru-hiker). I think in the end I'm happy we have options, as I trend toward trying to have a simple, relatively inexpensive backpacking kit, and I think MYOG is a great way to pursue this.