How much does a professional paint job usually cost. Changing the color from gray to black.

Hmm.Depends on what your gonna want.Maaco's ambassador service goes for $299.Visit their site for your preferences and better pricing
www.maaco.com/

Depends on the paint...acrylic-urethane costs more, but is stronger. It also depends if you want pearl, metallic, flake, etc.

You could get a $300 Maaco paint job.

Or have a more professional paint job for about $3000+.