There are dozens of variables, first what kind of paint. Catalyzed urethane, straight urethane, synthetic enamel, acrylic enamel, catalyzed acrylic enamel, Lacquer. The cost of those paints with solvent can be any where from $100.00 per gallon ( with solvent/Catalyst) up to $3-400 per gallon. Thats just starters. Do you want the car painted balck ....."as is" ( sanded lightly and prepped), or do you want the car prepared perfecly for a black paint job( rigorous sanding/smoothing/ realignment of panels perfect?????
Simply stated Maaco auto paint will do a black paint job for $500.00 that's about the cheapest......A custom shop will charge anywhere from $1,500 to $5,000 you choose what level of perfection that you want to pay for.
Having said that I picked the best custom painter in my area, and I prepped the car to the level that I felt comfortable with....He squirted the paint and color sanded/rubbed out for $500.00. But I did most of the hard work.