Here's my recipe...
1 Cup Washing Soda (not Baking soda). You can find this in the laundry section of most grocery stores. It's made by Arm & Hammer. It's sold as a laundry booster. Comes in a big yellow box.
1 Cup Borax soap powder. You can find this in the laundry section of most stores.
Combine. Add 1 - 2 TBSP to closed soap section. No need to add to pre-wash section with this detergent. It works well.
After using this for a while I realized that I was using too much (I don't measure. I just pour it in) and there was a white film on my dishes. To combat this (since I am still too lazy to measure), I added 1/2 C powdered Cascade to the mix. That took care of the problem.
I honestly don't know if it is cheaper. I never figured it out. It should be though because I have been using it for a bunch of months and have only had to buy another box of washing soda. I do like however that it is safer on the environment.