I've been making this for years. I use 2 TBSP per load of laundry, which costs $0.09.
1. Grate the bar of soap.
2. Add the soap, washing soda and borax to an air-tight container.
3. In a day or two, once the soap flakes have become dry and brittle, blend the whole mixture in a blender or food processor. If you skip the blending step, the soap flakes tend to clump together in the wash instead of dissolving.
Tip: Do the blending step outside if you can. The powder gets in the air, and it hurts your lungs.
cost per batch (2 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): $0.09
For comparison, I read that the average cost of laundry detergent per load is $0.21.