For night time temperatures 50-60°F you don't really need much, so my first thoughts are much like Bill's: are you sure that 50°F is really the worst-case nighttime low temperature you will see?

I usually tell people to invest in a good bag first and foremost when putting together a backpacking kit. Nothing else will last as long and get you so much more than a high quality sleeping bag. However, at that temperature rating shelling out big bucks is a bit of a waste. You could probably do this with a down blanket from Costco.