So, I'm going to Philmont (A large hiking area for boy scouts in the New Mexico mountains) and I have a cheap hiking sleeping bag from Walmart right now, it's warm to about 55 degrees and very light, It can get down to about 25-30 at night so I'm wondering if I'm better off getting a sleeping bag liner or a new sleeping bag for the trip.
You need a new sleeping bag. The marketing for sleeping bag liner would want you to believe that sleeping bag liners add warmth to your existing sleeping bag, but in my experience the difference, if any, is very minimal. You get more warmth by wearing a few layers in your sleep. If your bag is 55 degrees and you need to be comfortable down to freezing, sleeping bag liner will not do. You need a new bag.