Dawn dish soap works very well as a cleaner for the foam filters and won't harm the foam and yes it is much cheaper than the spray cleaners. Just be sure it is dry before oiling and reinstalling the filter. Some people saturate their filter in oil then squeeze the excess out, but I have found just spraying the outside of the foam then working it in, with the filter in a zip lock bag (to protect your hands) works even better. Never had any dust get thru, and you use a lot less filter oil.