Well, I went for it.
This is my first batch of laundry soap mix. I used a 2:1:1 ratio of grated soap, borax, and washing soda. I bought all the materials on Alice.com, which if you haven’t tried already, you really should. (Edited to add: here’s a link for a $10 Alice.com credit once you spend at least $50 there.)
I used about three tablespoons for a front-loading washer. The soap took longer to slightly dissolve in cold water — not so much that it left any soap behind, but I did see flecks of soap in the cold water wash when it went in. (In the warm water wash, the soap dissolved right away.) It was very low-sudsing, which gave me pause, but everything came out really clean. I noticed one of my husband’s shirts had a ring on the collar, which I would have treated if I had seen it earlier, but I just stuffed the shirt in the washer and hoped for the best — totally gone!
I bought 55 oz. of washing soda was $2.99 and the borax was $4.29 for 76 oz. Both of those will get used for other cleaning jobs around the house. I also bought three 3.5 oz. bars of this hypoallergenic Dial soap for $1.39. (I deeply regret buying the cheap soap, honestly. It smells, well, cheap — chemically and nasty. The smell doesn’t linger on the clothes, but still, blech.)
I don’t have a kitchen scale to measure exactly, but estimating by volume, I used about $0.33 of the borax and $0.32 of washing soda, mixed with one bar ($0.46) of soap, that’s… $1.11 for about 20 washes, or $0.06 per wash. By comparison, the Seventh Generation detergent I had been buying sells for $5.48 for 32 oz. on Alice.com, and is good for for 32 HE loads ($0.17/each) or 21 regular loads ($0.26/each).
Assuming my estimates aren’t wildly wrong, I think I could even switch over to less crappy soap and still be paying less. Hooray for being cheap!