Quite a challenge.

I use a 40 liter pack for 3 season backpacking, but I use a quilt instead of sleeping bags, and it is high quality down and outside your quoted price range.

Keep in mind - you get what you pay for with sleeping bags. The high quality down bags will cost you a lot up front but will last decades with proper care. Lower quality stuff won't compress as well, will weigh more (anything from ounces to pounds more depending) and you run a higher risk of finding the rating on the bag/quilt isn't accurate.

Also, if you get something like a Western Mountaineering, Montbell or Marmot bag, and find you don't like it - you'll be able to sell it used in a heartbeat.

Personally, I think the $240 I spent on my JRB 3 season quilt was the best decision I made back when I knew very little about the current offerings on gear - I will probably be using it for years to come. I can put both (I liked the quilt so much I got a second one to use under my hammock) side by side in the bottom of my 40 liter pack and have tons of room for bear canister, kitchen stuff, clothing, etc.