Use vinegar and hot water to clean your paintbrushes.
I wasted a lot of money having to buy new paintbrushes before I figured this out. I fill a small bowl with 1/2 vinegar, 1/2 hot tap water and soak my paintbrushes after each use for about 30 minutes. If your brush is in really bad shape, bring some vinegar to a boil and soak your brush in that for a few minutes. Wash it with some soap and water afterwards, and your brush will look like new.
In the below picture, the left brush was just soaked in water, and the right brush was soaked in vinegar and hot water.