I have 4. All of them 2nd hand. Total cost is under 200usd
The first two are a military liner (1.5lb) and sleeping bag(3lb). Both synthetic mummy bags. I use the liner when it's going to be 55+f all night. Essentially only nights where I'd be sleeping with just a sheet at home. I've also used it when I cant spare the bulk (hitchhiking) and tough out the misible sleep with some tricks*. The large bag I mostly use on river trips because it dries faster than down. Its warm enough until about 40f.
Third is a 3lb down mummy bag.This is my recommendation. Really good for a dry fall night down to into the upper 20s f. Lighter and less hassle than wrestling a bag and liner together before bed. Packs really small for the comfort. If I could only keep one this is the bag the bag.
The last one in a 4.5lb envelope down bag. Its enormous and I really only use it car camping when hunting in the mountains. I've pushed it (in the shell of a truck) to single digits f comfortably. It's as thick on both sides as a down comforter, but since its seemingly 100% down (no feathers) it's much lighter. Still too heavy for all but your coldest trips, the kind that could actually hurt you.
*a few things can really stretch the usable temps for a bag
-BONE DRY wool socks, not the ones you've been wearing all day. Yesterdays only if you dried them completely on a hot rock in the sun or by the fire
- ABSOLUTELY dry long johns, I like fleece
-a knit cap
(I keep the above 3 things in the bottom of my sleeping bag when I pack if I'm risking going light)
- I use a $15 target puffy jacket which isnt too bulky to sleep in.
-a water bottle filled with hot water, or the ol' hot rock trick if you dont mind stubbing your toe all night.
-eating right before sleep
-take a piss right before bed, its less mass to keep warm
- if you're a side sleeper get an inflatable pillow. Seriously, you lose a ton of heat having an arm stuck under your head and half out of your bag.