Fillers that would actually be valuable additions to your soap would be things like washing soda or borax. They add detergency (cleaning power) and would be appropriate additions to a laundry powder.
Other fillers that I can think of, such as the potato flour that Obsidian mentioned, are not going to add to the quality of your soap. They are ingredients that only add bulk and weight to your product and thus reduce its cleaning power. Even water can be used as a filler.
From "Small Scale Soapmaking" by Peter Donkor, 1986, which was written about soapmaking in Ghana:
"...Fillers are used to add weight to the soap without in any way adding to the detergent property of the soap. They increase the bulk of the soap, and hence reduce production cost. They are not, however, used in good quality genuine soaps. A number of filling agents are used as fillers, but the most popular ones are clay, kaolin, talc, starch, common salt, chalk and magnesium carbonate. Soda ash and sodium silicate are also used in large quantities as fillers - sodium silicate when used, also acts as an antioxidant to protect the soap from going rancid. It also improves the smoothness, binding, transparency and hardness of the soap. In the use of fillers, care must be taken for their selection and quality as too much may dampen the washing properties and keeping quality of the soap...." (pages 19-20)
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Another way to reduce costs is to look at alternative cheaper sources of fats. Susie mentioned talking to butchers and meat markets for fat from butchered animals. Another source of fats is any place that serves food -- they often have waste grease that can be collected, cleaned up, and used for soap.