The comforter may be filled in with cotton, polyester or other fibers such as natural silk.
Most comforters filling in, or if not clearly addressed, are polyester fibers. Except the quality of fibers, the polyester filling in also can be weighted from 80 to 400 gsm. (Grams per square meters). The more gsm, the thinker is the comforter. For most of bed coverlet kind of comforter (or bedspread, quilts), the filling in is 80-120 gsm. They are good for summer or an extra layer during winter. Except in very warm surrounding, this kind of filling in is not good enough for winter.
It is very popular that comforter has 220-280 gsm. Those comforters are good for spring, fall and winter with warm surrounding. They are the most common filling in you can see in the market. If you want to very warm comforter, you will need to consider filling in over 300 gsm. It is not that popular in the market so you may need extra blanket combined together.
The gsm of polyester filling in has not too much to do with the comforter cost. The most important parts are comforter facing backing fabrics.