The age old problem with bags - Your Warmth May Vary.

But my general rule is that if it costs less than $100, it's probably not going to be warm enough for me, and definitely going to be too bulky for my taste.

In sleeping insulation and computer hardware - I don't cheap out because it will be more expensive for me in the long run to try to use budget gear. As an end user I have fairly high expectations, and use the gear often and hard.