An excellent question! Certainly, if cost alone were a factor, then synthetic sleeping bags are the easy winners. Bags in the 15-degree range, made of material such as Polarguard Delta, typically cost $150 to $200, depending on features and size. For example, Mountain Hardwear’s Second Dimension is a good quality bag that sells for $179.
Due to higher material and manufacturing costs, down bags are more expensive. Prices range from about $200 to almost $400, depending on a bag’s features as well as the quality of the down used. A good mid-range bag is The North Face’s Hot Tamale MF at $319.
Against price one must weigh three considerations: weight, comfort, and longevity. Although synthetics have closed the warmth-for-weight gap, they still lag behind down. The Hot Tamale, for instance, weighs a half-pound less than the Second Dimension even though it’s a larger cut than its synthetic rival. Down bags also stuff more compactly, therefore occupying less room in a pack. And overall, I find that down bags drape over your body more comfortably, providing a snugger fit than stiffer synthetic bags.
Finally, longevity. A good down bag, if kept reasonably clean and packed loosely, shouldn’t “wear out.” I have down bags that are ten years old and almost as good as new. Synthetics, meanwhile, tend to break down. The material itself loses loft over time, and the flat batts of material that go into a synthetic bag can separate, causing cold spots. How long does this take? No clear answer, but in most cases a synthetic bag can be expected to weather four or five years of moderate use before one might think about replacing it. So for value聴or dollars spent per year of a bag’s life聴down is the winner.
Of course, all of the above has to be weighed against a synthetic bag’s one great advantage: performance in wet conditions. Polarguard, Primaloft, and other synthetics lose little insulation when wet and they also dry quickly. Down bags, once wet (it actually takes a surprising amount of moisture to really soak a down bag), are next to worthless and take forever to dry.