Swiffer Duster you know the replacements are expensive. Making a replacement is quite easy. I used t-shirts, but many recommend using fleece, which would probably work better as it's thicker, but it's counter-productive to purchase material instead of using what you have. The t-shirt material seems to do the job and I added a few old dryer sheets between the layers of t-shirt to attract more dust and make it stiffer. It works fine. Next time I'll add a few more layers though.
Cut 2 (or more) pieces of material 7" x 5". Place on top of each other and sew down the center. Sew 1" on either side. Cut from the side to the first line of sewing on each side (at about 3/4" intervals). Insert the prongs from your Swiffer Duster and you're good to go.