A bag of 40 diapers costs about $10, so you will be spending about $.25 each. You can reuse these a couple dozen times easily, so figure on a penny per use (not including any energy cost for running your microwave, of course). Not bad, when compared to the $12 for one or two single-use disposable hot pads at the drugstore. These retain thir heat for a couple hours.
Step 3: Fill with water.
Whichever method you use, try to get the water absorbed to all edges and press it around a bit so that it is nice and puffy end-to-end.
Step 4: Enclose in freezer bag.
I fold the outer edges inward to form a neat tubish rectangle and then stuff it into the bottom of the freezer bag. Again, squeeze it around and work it into the shape of the bag.