Well the theory was that most sub 4lb bags have one baffel system, that the baffels trapezoidal box etc are similar. They mostly have zipper baffles and hoods.
The actual fabrics are important, but most are expensive some are quite expensive indeed, but not to the point of serious money.
The mountain equipment bags are probably sewn in the same place as the vaude bags etc ie a factory in china.
Even the price discrepancy between the same manufacturers model at a different temerature ratings gives you an idea of the expense of down, and all they have to do is take a bigger handfull and ram it in there. I should not think the difference in model ratings is due to the addition of an extra baffel layer in most cases unless you select an arctic 8lb down bag.
Its just that there seems to be alot of cheaper bags, then the starting range of about 500fp down by well known manufacturers. The 850fp goose down seems to be about the best you can get generally, duck down being less fp.
Take this bag
https://www.essentialemergencygear.com/vaude-ice-peak-400-sleeping-bag/Thats 500 dollars on a 650fp bag. I think there are equal teperature rated bags with 850fp for around the same price.