I've always been a huge fan of Snugpak for high quality budget sleeping bags. The Travelpak 2 which is pretty small and light, and is good down to ~27-32F, is $39 shipped on amazon. The Travelpak 4 goes down to ~14-19F, for $53 shipped on amazon.
Paired with a USGI goretex bivy ($50ish on ebay), I've used these below their "extreme low" rating, but I handle low temps pretty well, and women run colder than men, so YMMV. Plus then you have a windproof/waterproof shell. Unless your pack is gigantic, you can probably stuff it in the bottom of the bivy and keep everything dry.
Might be more than you want to / are able to spend, but if you can afford it I think it's a pretty great combination.