A Seattle mail order merchant called "REI" and made his own sleeping bag. He figured that having made the bag, he'd know how to open the right seam to replace damaged or lost down over the years.
Isn't it amazing what REI has morphed into? But yeah, once you have the core shell set up, it's trivial to purchase super-high quality down. T-H sells 3oz baggies of 900FP for $45.
The quilt pictured upstream - and yeah, it's also enclosed in my pack photo above, along with a BV450, so you can see how compressible it is - is filled with 12oz of 900FP. But, it would take me maybe something like 5 minutes to tear open a seam, sift in 3 more ounces, and have a sub 25 degree quilt. (I think my range right now with 12oz is around 28-32 degrees.)
The M90 material is really high-tech - the stuff weighs less than an ounce for a square yard. So, do the math: 12oz of down, 5.6oz for the material equals 17.6oz. The flipping stuff sack weighs .7oz alone for comparison (probably the cord lock LOL) for a grand total of 18.3 ounces.
Technology has advanced in so many areas, including material science. The whole discipline of understanding CLO values provides a lot of answers to helping design appropriate gear using the most modern materials to achieve really high strength/warmth/dry to weight ratios.
REI and other mass market retailers are way, way behind the curve on this stuff. There's a handful of cottage gear manufactures (like Steve's Zpack) that are pushing the envelope, but it's so much easier to just whip the stuff out yourself if you have some basic skills.