Don't be afraid to pay $20 for a good paint brush. It will make painting SOOOO much easier, produce a nicer finish that minimizes brush lines, makes cutting sharp lines against a ceiling or around trim a lot easier, and if you clean it out properly after each use it will last a long time.
I only use cheap brushes when I have no intention of cleaning the brush when I am done. Like when using a shellac based primer that requires methyl hydrate to clean the brush. Throwing away a $2 brush is cheaper than cleaning it.