More expensive will be better performance..
But the rule of thumb is :
if he uses OpenGL programs - buy quadro/FirePro line, but if he uses Direct3D software - buy gaming card.
AutoCAD uses OpenGL, so Gaming card will be useless(gaming cards have blocked true OpenGL performance, or pro cards have somehow accelerated, so difference is big)
Quadro 6000 is the most powerful workstation GPU from nvdia. excellent in OpenGL, weak in Direct3D
FirePro w9000 is the most powerful workstation GPU from AMD. excellent in OpenGL, very good in Direct3D
I suggest you to use AMD cards, because it can work flawlessly in both mode, application (OpenGL, Direct3D)