I'll offer a few options to play with.

You might try using a fleece bag inside your down bag. Coleman has one for about $20-$30.

You could try using a piece of bubble foil insulation under your bag. I use one and they work good for my skinny butt.

You could try using a SOL emergency blanket over your bag. I use one of those too, and it works for me as well.

On really cold nights I use a couple "Hot Hands" packets in my bag. I don't think you'd want to carry 32 of them on your trip, but if you have a few for those nights when your still cold, they're pretty sweet.

If you build a campfire where you'll be going you could always play with using a campfire tent. The one I made last year works good, even if you don't have a campfire. (Here's a thread discussing it with some photos.)

If you could squeeze by with cheapie options like these for another year you might be able to save up for the Western Mountaineering bag