Fabric softener isn't incredibly expensive, but chances are you already have all the necessary ingredients to make your own just lying around the house.
As ChronicCrafter explains on Instructables, all it takes is water, bicarb soda and hair conditioner. If you're comfortable using vinegar in your laundry, supposedly that will help as well, but I'd test it on a load of rags first.
Simply mix the ingredients in the proper proportions, store the solution in a well-labelled container and use as needed. It might not save you a lot of money, but it might help you avoid a trip to the store if you're caught without any fabric softener at home. Hit up the source link for the complete recipe.
Make Your Own Fabric Conditioner [Instructables]