Sleeping bags... you get what you pay for. The "cheapest" sleeping bag that is warm, packs down well, and accurately rated - will not be cheap. Something like a Western Mountaineering - not a small price tag, but it is the bag of choice for veteran backpackers who go a lot and want something that works AND is the lightest and most compact solution as well as (reportedly) warmer than rated.

i am a quilt user in three seasons and have a Marmot 0 degree for snow camping. I spent about $500 on two down quilts from Jacks R Better to use with my hammock, and that was six years ago - still using both quilts and still happy. My sleep system depends on the expected conditions I will be in. I use a quilt on a Big Agnes Q Core or on a NeoAir with good results when tent camping, and both quilts on a hammock when below treeline. As a side sleeper I've found that inflatable insulated mattresses are the only way I can be comfortable - nearly as comfy as when in a hammock - on the ground. A little inflatable pillow helps a lot on the ground.

Also part of sleeping well and warm are your eating and drinking habits - sleeping well has to include how you fuel your body, which is what heats you up all night. Having adequate calories and plenty of water makes a big difference. I've backpacked with folks who stop drinking so they don't have to get up at night - that's a mistake.

What you wear to bed also matters. The colder it is, the more wet/damp clothing affects your comfort. Changing out all the damp (sweat) clothes you hiked in can make the difference between sleeping warm and sleeping cold. I take a base layer and will wear my spare pair of wool socks to bed, changing back into hiking clothes in the morning.

Caveat - quilts don't work for everyone, but Jacks R Better makes big ones for couples or larger people, and many cottage gear vendors make custom jobs. And Western Mountaineering has different sizes of bags - width, length - to help folks of different dimensions find the one that fits them - many cheap bags are one size fits all.