had occasion recently to sleep in my 3 season down bag. marmot sawtooth, 600 fill power down. weighs a little under 3 pounds. a very good, very happy sleep. the bag stuffs into a reasonably small sack and is good to 15 or 20 degrees. i have had it a while, cost me about $150.
marmot's high-end 15 degree bag, the plasma, weighs one pound less. it's good to about the same temperature range. 875 fill power down, ultralight nylon. it costs 3-4 times as much as the sawtooth (based on a quick look at campmor and REI's websites, anyway). stuffs into a smaller sack, no doubt. a fine piece of gear.
ps - not knocking the brand, but if i were ever inclined to throw north of $500 at a spring/fall sleeping bag with 850 flll power down, it would probably be from feathered friends, western mountaineering, or valandre. beside the point, because those are also quite expensive.
throwing out the question: is it worth 3x the cost to save one pound and a little space?