1 Fels-Naptha bar (97 cents in the laundry aisle at Wal-Mart)
1-2 cups Arm & Hammer Super Washing Soda (sodium bicorbonate) $2.75 for a big container
1/2-1 cup Borax $3.25 for a big container
1-2 capfuls Oxi-Clean Max Force Liquigel $9.00 for a bottle
All of those ingredients are found at Walmart and cost approximately $16 for all four items. The only item you buy each time you make 5 gallons of detergent is the Fels Naptha bar. Once in a while, you'll have to replenish your other supplies, of course, but this is a huge money saver.
First, put on a large pot of water to boil (about a spaghetti boiler's pot full). While it's boiling, grate the bar with a cheese grater. I'd recommend getting a grater from Dollar Tree just for this purpose. Once the water has reached a full rolling boil gradually add the grated soap, stirring constantly, until it's completely melted.
Fill a five gallon container halfway with extremely hot tap water. Pour the melted cheesewater into the tapwater and stir. Add in the washing soda and Borax and stir thoroughly until completely dissolved. Add the Oxi-Clean, mix well, and let sit overnight.
The only other thing I can think of to say about this detergent is that it's a smart idea to stir it occasionally, and make sure you shake well before you add it to your washer. Other than that, it's the best laundry detergent I've ever used. Feel free to add essential oils to it to make it your own.