Cheap brushes mostly shed hair on your painting and lose shape and become useless very quickly.
This is mostly due to cheap hair, bad assembly, poor glue. the other factors are users' abuse and medium and cleaning agents effect.
A good quality brush that is handled correctly, cleaned and stored properly lasts for life...for cheap bruses i have seen some, even some cheap red sables, disintegrate after a single use.
Until your skill level warants using $50 plus sable brushes, buy synthetics. Even synthetics have a greet range of quality and price.
For good brushes generaly avoid cheap chinese hog, cheap poney and cheap synthetics.
As a rule of thumbs, the price of a brush should be related to a tube of paints you use.
For instance to get a genneral idea relatively in euros, £, or in $
- $1 kids paint, $1 poney brushes
- $2-4 learner's paint, $ 2-4 bristle, craft grade or cheap synthetics
- $4-10 artist grade, $ 4-10 good synthetic and some hair range like kevrin
- $10-50 great fine art paints, $10-50 top of the line brushes.
So when you see supermarket brushes at $1 for 3 brushes pack, or $4 chinese hog bristles 12 set...run away and paint with your fingers.
That being said there are some exeption, some cheap synthetics can sometimes be quite good.