Coconut oil is a popular base for homemade toothpaste - this is how I use it:

With a clean spoon I transfer some solid state coconut oil (as opposed to liquid / melted) onto my toothbrush, then sprinkle that with a pinch of baking soda.

That's it!

The coconut oil is known to have its own benefits, but it's the baking soda that neutralizes odors. This formula will leave your mouth clean, but not minty fresh. For minty fresh, you can sprinkle a small amount of ground, fresh mint over the layer of baking soda, or just chew on a couple of fresh mint leaves after your brushing routine.

Compared to what you could spend on toothpaste, coconut oil and baking soda are far more affordable by volume. Note that coconut oil has a shelf life that can be impacted by how you store it, so buying a giant tub for one person might lead to more waste than savings.