To make your own or not. There are tons of recipes on the internet for homemade laundry detergent. Some even use left over bar soap scraps. This is a great way to save money. I have never tried it, but I have a few other ways I save on laundry detergent. First, I never use as much as is recommended. Most of the time I use about half the recommended amount and my clothes are always clean. The other simple money saver is to add a box (or two) of baking soda to a box of powdered detergent. It stretches the detergent and keeps clothes smelling fresh.
And I always wash a full load and in cold water......but you already knew that!