Think about how cold you get when you sleep and buy accordingly. Prices can vary greatly from store to store, so shop around. If you have to buy just one expensive thing, this is it, guys. You could use a pad made for the purpose, or foam padding, or a chaise lounge pad, or even a folded blanket or two. Last but not least, a few items to think about.
Heat packs are a great, inexpensive asset during the cold months.
I always have a few in my bag. They can be purchased at Big 5 or a backpacking store. You just shake them up to start the heat rolling, and they get pretty darned hot. Use them in your pockets to keep your hands warm whenever you feel the need. Remember to bring hot beverages.