I use baking soda instead of toothpaste and add a little water. Would it be okay to mix the baking soda with my (leftover) mouthwash, just making it moist, and calling it "toothpaste"?
Making your own toothpaste is both easy and cheap.
Homemade Hair Dye Recipes
Using Toothpaste to Clean Silver
Saving Money on Toothpaste
Homemade Tooth Powder
Cleaning with Toothpaste