Swallow hard, spend the money, and get a decent,properly rated down bag, especially since you list compactness as an important criterion. It isn't easy, but you will be grateful for years into the future, when you spend warm, comfortable nights in the bag. With the liner, you can probably get a bag rated to 20F or so.

Prowl second hand stores - you can find good deals there if you shop carefully, or try REI or other major retailers, and see if you can get a discount.

What is the size of the bag you are using?

Remember,too, that sleeping warm and comfortably is more than just the bag. What kind of pad do you have beneath you? Are you eating a meal with complex carbos that will continue to heat you into the night? Is your campsite well sheltered? These are all critical factors.

You will get lots of opinions, and there are lots of bags out there. Offhand, I would look to the REI bags for quality and reasonable price. You will doubtless get other opinions.

Warm, compact, cheap - Pick two out of three.