I would be quite suspect of any silk liner below about £25 (unless in a sale) - it's one of those 'too good to be true' issues for me. I have a Rab one and that has had quite a workout with no quality problems, etc. A cotton liner is ideal for warm and dry conditions but a silk liner is a must for cold nights. It's not just about warmth - it will also keep the inside of the bag cleaner so it lasts longer and will need to be cleaned less often. I have used mine for climbing, mountaineering, military exercises and ops, adventure racing and mountain marathons, where you often go to bed dirty (!).
I'm convinced (through experience) a good silk liner will help extend the life of your bag - especially down bags - and is one of those items where a quality buy will pay for itself through years of use.