My sister has been making her own laundry detergent for some time now and I decided to hit her up for her recipe. She had broken it all down in to some obsurd amount that it ended up costing per load (I think about .6 cents as opposed to about .25 cents for major brands). I was all hyped up about trying it out and making my own. 
She sent me the list of ingredients (all 3) and said they could be found in the laundry aisle at Walmart. This only posed a small problem which is that there are no Walmarts in West Los Angeles. Okay there is one 8 miles away but it's super ghetto and I avoid it like the plague. The next option is about 15 miles away. I went. They had the Borax and Fels Naptha. No washing soda. So I head home, go to another walmart 15 miles away another day and still, no washing soap. Obviously at this point, it's not cheaper for me to make my own because I can't find anyone who carries the Arm & Hammer Washing Soda. I tried pharmacys, grocery stores, home depots, etc.,. It was ridiculous that I couln't get my paws on this magical potion. Finally, I found a little hardware store on Sepulveda (if any of you locals are brave enough to try your own :) and purchased a box for about $5.00. My sis says she paid $2.98 at Walmart so the cost of making this yourself is really going to depend on where you live and how easy it is to track down the ingredients. I paid .97 cents for each bar of fels-naptha, $2.98 for the box of borax and $4.98 for the washing soda. 
Here's what you'll need:
Box of Borax
(4) bars of Fels Naptha soap
(1) box of Arm & Hammer Washing Soda (Not baking soda)
Directions:
Start out by grating one bar of soap with a cheese grater & place it into a cuisinart or blender. Add (1) cup of borax & (1) cup of washing soda. Pulse/blend until well combined. Store in an airtight container. Use 2 Tablespoons per full load (1 Tbsp for 1/2 loads). The soap will not foam up like your standard laundry soap but it's still working!
I quadroupled the recipe to get the most use out of the ingredients.
Because I don't have a washer/dryer in my apartment and have to carry the soap out to the laundry room, I chose to store it in Kerr canning jars. If you have laundry in your place (I'm jealous) you can store it in one big container. 
Let me know if you try it~
Happy washing!
XO
(p.s. let it be known that laundry is the chore I loathe/dread the most)