Of course there are many considerations but as a "rule of thumb" what are you willing to pay per ounce to reduce pack weight?
Example: I have a warmer weather Nature Hike LW180 bag that weighs 1.5 pounds with it's compression sack. I can spend $75 to get an Aegismax down bag that weighs 1 pound and 1 ounce with compression sack. This results in a 7 ounce savings which comes out at $10.71 per ounce. The Aegismax offers a slightly smaller compressed size and a little lower temp rating too.
Deal or no deal?