I've been making this for years. I use 2 TBSP per load of laundry, which costs $0.08.
1. Grate the bar of soap, and store the soap flakes uncovered for a
day or two so that they become dry and brittle.
2. Once the soap flakes have become very dry, grind them to a powder in a food processor (or blender). If you skip this step, the soap flakes tend to clump together in the wash instead of dissolving.
3. Mix the soap powder with the other powders and store the finished product in an air-tight container.
cost per batch (2.25 cups)
bar of soap: $0.40 (I bought a 10-pack of Ivory soap at Target for $3.99.)
washing soda: $0.70
total cost per batch: $1.44
total cost per load of laundry (2 TBSP powder): $0.08
For comparison, I read that the average cost of laundry detergent per load is $0.21.