There are two different types of filling that you can get inside a bag, natural down and there’s also man made. The down sleeping bags are actually more expensive than their same yet man made products, but it will last longer and keep you warmer as well. The down is made from the feathers of a duck or a goose, and is measured at a rating of cubic inches per ounce is much better and more expensive as well! The synthetic filling used in most modern sleeping bags is also good at creating you more warm and comfortable and it will cost you much less than a down bag. The only downside to this is that the bag will not keep you that warm just like a down sleeping bag would and it will not last for a long time, but it may be better to buy for someone who is not using it that much.