If you go with a lower-end bag, you may want to allow a tiny bit more girth (i.e., don't get a bag that fits snugly.) This will allow you to extend the temperature range of the bag by wearing fleece or down garments inside it. Don't get a loose bag; just allow enough extra room for the thickness of a layer of insulating clothing. This gives you a safety margin for any optimism that the manufacturer may build into its rating.