dishwasher soap - Baking Soda and Borax. Mix together equal parts of each ingredient, store in a labeled container Use just like a commercial powdered detergent, approximately 2 TBS per load. That’s it! Each load will cost you mere pennies.
For us Southern girls who grew up being told to NEVER EVER wash your cast irons with soap...scrub your cast iron with coarse salt and a soft sponge. The salt is a natural abrasive and will absorb oil and lift away bits of food while preserving the pan's seasoning. Rinse away salt and wipe dry.