If you haven't seen
Em's excellent videos, do yourself a favor...
I have a $10 Omega Professional boar hair brush I bought from my barber a few months ago, but haven't used much because I found it too stiff. I decided to give it another go this morning - but this time I shampooed it with some neutrogena cleansing shampoo, which is very good for removing grunge, then conditioner. Soaked it in my scuttle while I showered... and what a difference! :)
Something to try before you give up on your cheap brushes. Or to make the most out of your nice ones.