Of course, it all depends on what you're using the bag for. You can get a cheap 5lb 45F bag at *-Mart or Target for under 20 bucks. You're not going to want to carry it on your back for long, though, so you better be setting up in a campground. Or, you can spend $250 on an ultralight bag that'll keep you warm when it's -20F.
I personally recommend Target's <A HREF="http://www.target.com/common/catalog/Product.jhtml?prodid=27667&catnav=21706&subcatnav=21752">Grand Canyon</A> bags. They're 1.25lb 30F ultralight bags, and at $40 each, they're insanely cheap. I've got a couple of them, and I love them. They weigh next to nothing, and compress really really tight. As ultralights, they have a nylon shell, so you might want to get a fleece liner if you're not doing any hiking, since bare nylon isn't that comfortable to sleep on.
BTW - the best thing about this sack - no compression sack! There's an extra foot of nylon at the end of the bag that wraps around it, and uses straps and drawstrings to make its own bag. Much easier than screwing around trying to roll the bag tight enough to fit in a stupid way too small sack.
[This message has been edited by jclark (edited 07-21-2000).]