I just painted my 400 with 1 pint and had some left over. It was $62. Now, that's just the color. You need hardener and reducer, too - I already had that.
That's if you only have a single stage paint.
With a 2-stage ( base coat/clear coat), you need hardener and reducer for each. It can run into some money.
As greginnm said, depending on the type sprayer you have access to is another consideration. A conventional sprayer (with a compressor) uses a LOT more paint - it blows it into the atmosphere trying to atomize it. I use a true HVLP and it uses 30% less paint than conventional.
to answer the original question, you will probably have to buy a quart of color. Then, the paint store will sell you whatever you need for the particular paint you get.