Sorry for the delay in replying, I didn't get a forum email about this post (which is a bit unusual but not unheard of).
I use several different bags, it depends on what I'm doing and where. They're all synthetic, washable.
My lightest bags are the tropical types, I have an ex-army one that was given to me with a load of other surplus stuff, and a one-season thing that I picked up in Oxfam for a couple of quid.
I'd use one of them in a bivvy bag if I was touring on the bike in the warmer parts of the world in summer, or if I was doing a long run and expected to break the journey overnight.
I have a polyester bag liner kicking around somewhere and it's useful for an extra couple of degrees but I don't rely on it. Another charity shop find, unopened in its little stuff-sack, again a couple of quid.
My medium bag is a Coleman mummy style which is about ten or fifteen years old now. It was originally alleged to be good for minus 18C but I never believed that for a moment. I'd call it three season now and I'd use it in an issue MVP bivvy in mild winter weather like in the UK. It weighs just under two kilos.
I don't know what a similar bag to the coleman would cost nowadays, I won't guess as I could be way out. Back then it was well over 100 quid. You can pick up an excellent used issue bivvy for thirty quid and they are first class bits of kit.
My cold weather bags are a Jerven bag ('Thermo Extreme') which I often use with the Coleman bag inside it, and the US Army cold weather gear that I mentioned earlier which is two sleeping bags and a MVP bivvy cover that all nest nicely together. Mummy style.
Either of the cold weather setups will cope with severe cold but the Jerven bag is more flexible than the US Army bivvy and I tend to prefer it for short trips where I'm not going to spend much time setting up.
They're both on the expensive side, and a bit bulky. The US Army thing is very bulky and probably weighs about five kilos. My electronic balance is bust at the moment or I'd weigh it.
If I'm going to be able to get properly settled then the US Army gear suits me better - although it's not so roomy as the Jerven bag and I do like having some legroom. The Jerven bag doesn't breathe like the bivvies so it needs regular airing.
Finally I have an old (Czech?) Army bedroll which I really rate for mild weather. It's a rectangular nylon cover with a blanket and sheet folded into it and held in place with buttons. The cover has a zip and it's waterproofed on the bottom. It's heavy. I use it at home.
It was with another job lot of surplus stuff, didn't really cost me anything. Actually I lied when I said they're all synthetic, the sheet in the bedroll is cotton.
Does that help?