I have been making my own laundry detergent for some time now. Here's my recipe:
1 cup Fels Naptha Bar Soap grated
1/2 cup Borax
1/2 Washing Soda
Mix and store in an air tight container. For small loads use 1 tablespoon and for regular loads use 2 tablespoons and extra large loads use 3 tablespoons.
I have found that this costs me about .10 per load. Regular laundry detergent costs around .25-.30 per load. The main reason, however, that I make my own soap is that it is extremely easy on my skin.
You can also make liquid soap if you prefer. Here's a great site to find out more. I also love, love, the product they sell called soilove. Enjoy!