If 80% of your outings will be done when a 30-40 degree F bag would do, why not pick up a reasonably priced used bag for that temperature range, and use your existing heavy bag (perhaps in combination with the "new" one, to handle the rarer outings in colder temps. I've bought 3 good, lightweight, used synthetic bags, all for under $50 each. The REI Down Travel bag (http://www.rei.com/product/746296) is small, lightweight (27 oz.), and runs $109 new. I loved mine when I had it.

Then later, with more experience under your belt, and recognizing what gaps you want to fill, you could shop again for a higher end, higher priced bag that would perhaps handle lower temps.

I just posted this on another thread where the guy was asking about lightweight:

Going lightweight is a journey, and an evolution. Among others, I own a Western Mountaineering Megalite, and a Nunatak Arc Alpinist. They are the "creme de la creme".

If budget is an issue, I would recommend first buying a used decent quality bag on one of the many gear forums. With the experience gained from many trips, you could later buy one that you feel is better suited to the areas and seasons you visit. Someone else will want your first bag if it is a good one, and has been taken care of.

At some point in time, you may have a collection that you choose from depending on the anticipated trip needs.

As many as I've now got, I still have desires for some others to add to my collection. I'd like a lighter (than 21 oz.) Nunatak Arc Edge (11 oz.) or Arc AT (8 oz.).

Good luck, and happy hoofing.