Room Treatment, Panel Sound Diffusers

So I am setting up a third sound system in a spare bedroom.  I want to really get the walls and perhaps ceiling treated for optimal sound.  The bed will be taken out.  I will put in high quality duplexes like the ones sold by Cullen Cables.  However, there are a bunch of different sound diffuser panels with many being quite expensive.  I am going to do the install myself so nothing too complicated.  Looking for advice on the type of wall treatment that works but won't break the bank.

Thanks!