Rather than a bag liner, I wear silk baselayers. I was considering a silk liner, but for one thing, it's a single use item. If that single use it important, and nothing can be substituted, then I can live with it. Another concern was that I read comments from restless sleepers (I am one myself) that they'd get kind of tangled in the liner.
My silk base layers are made by Terramar. They're usually available on Sierra Trading Post, and if you buy them during one of the gazillion sales they're always having, you could probably get a pair for around $35-40 shipped. My mediums weigh 6.5 oz total, vs probably 4-5oz for a liner. They also have the benefit of being dual use. Since they aren't worn on the outside of my clothes, I don't have to worry about them bringing dirt into the bag. And your main concern isn't dirt so much as body oils migrating to the down and reducing its effectiveness. I'm pretty sureI read that even if you wear the the silk baselayers around camp and sweat in them they still prevent this.