I think US-made bags are all pretty expensive. The WM Summerlite you mention is a great lightweight bag (I have one) but it cost more than I can usually afford for a piece of backpacking gear.

I wonder, though, if you might be a bit optimistic about your temperature-range requirements. Since you live in Colorado, I imagine you'll be in the mountains, and temperatures could easily fall to freezing, even in mid-summer.

As far as goose vs. duck vs. synthetic...good duck down is fine, it's just slightly heavier per unit of volume than good goose down. And synthetic is a bit heavier yet. So, "what fill power do I need?" really isn't the question, since the fill power is just a measure of how light the bag can be for a given temperature rating.

It's worth noting that a good down bag can last many years if taken care of, but synthetic bags lose their loft within a few years even if you don't use them. So the down bags are probably a better value in the long run.