Hmm... you haven't listed how much is the maximum you can spend. Kelty makes some decent light weight down sleeping bags which are often on sale at sierra trading post and campmor for just under $100.

Wool blanket + warm clothing is something that could work.. but I would encourage you to try it before you take your trip, maybe in your backyard when
the night time is likely to be the same as what you expect to be the coldest on your trip. The other possibility is to find a high loft bed quilt at a thrift shop. They won't be as light weight as down quilts designed for backpacking, but one made from a twin bed can be reasonably light and quite warm at a reasonable price. If you haven't tried it before, you might want to experiment with sleeping sitting up

The Str Rock $300 challenge is a good place to get some ideas. I also collected some idea on my backpacking for cheap page