I mix paint at the auto parts I work at. Depending on the kind of paint (urethane, enamel, etc.), the style of paint (metallic, micah, or pearl), and the color, prices vary significantly.
Typically, red is the most expensive, due to the cost of red pigments. Followed by oranges and yellows.
As Jeremy said a quart will spray boocoo guitars. A 1/4 to 1/2 pint is plenty. Just remember to make room in the paint budget for reducers, hardeners, and clears.