Castor is less expensive than synthetic - about 2/3 the price. Manufacturers use synthetic not to save money but because that is what modelers want. Running castor does tend to gum up engines and castor residue is harder to clean off airplanes than synthetic. But if you don't mind those inconveniences then you can save money by running castor.
Many commercial fuels are a blend of synthetic and castor. It is supposed to give the best of both worlds. The synthetic prevents gumming and the castor adds high temperature protection. I have noticed myself that a 50/50 blend totaling 20% will not gum up my carbs in storage nor deposit brown gunk on my mufflers. But running just castor does both.