We are lucky now to have the wide variety of art materials available for almost every conceivable purpose. In earlier times artists often had to create their own tools or adapt what they could get.   James Whistler would take apart brushes to remake them the way he wanted. 

There is no reason to ever throw away a brush!  I have probably hundreds of brushes, some decades old, that are still in use.  When a wide brush gets worn and splayed, I can use them for painting grasses or other textures.  When a brush is bent or misshapen, it is often useful to trim it shorter for applying frisket.   When a brush no longer has any useful hairs, there is always the handle!  You can use a brush handle as a stick for mixing or put the back end in a pencil sharpener and use it for applying thin lines of ink or even frisket.

A palette knife is useful for scraping watercolor or applying color with the edge. An old credit card, cut at an angle is good for the same thing or used as a stencil. I like to save old plastic lids from containers to mix paint or to cut a stencil. Plastic plates or bowls are good for that too since they are thin and easy to cut with scissors or an exacto knife.

Recycling art supplies not only saves money but the main value is in creative thinking.  Imagining what kinds of tools might be useful for a particular task or adapting your own is creative thinking that saves supplies and expands your artistic horizons.