Here's what I do:
64 oz. box of baking soda
55 oz. box of washing soda
76 oz. box Borax
3 bars Kirk's Castille soap
I grate up my bars of soap in my Cuisinart then mix the baking soda and Borax together with it in a big bowl. I then pour my detergent through my canning funnel on top of my old pickle jar. I use about 1 T for each load, but if it's a light load (rarely happens around here!) then I'll use a bit less. Sometimes I'll pour in a little extra Borax if it's a really dirty batch (like dirty, sweaty outdoor work clothes), but most of the time 1 T. will do it.
I use vinegar for my fabric softener. Less than 1/4 cup will do just fine for a regular load. The clothes don't smell like vinegar after they've been dried, either.
I use Kirk's Castille soap because it's a natural soap and great for the sensitive skin of a few of our children.
Total cost: less than $10 for the bar soap, the baking soda, and the Borax. That's less than $1.70 a month for our laundry detergent! And figuring that we usually do about 12 loads a week that's about 3 cents per load!
*I edited this to add 1 55 oz. box of washing soda. I found it got clothes cleaner and made the recipe go further. Those bars of Castille soap are expensive!