It usually depends on the kind of paint and the coverage you want. Full exterior and doorjams/etc I assume? But let's say you just want a good urethane....

1 Gallon basecoat. Remember, that gallon gets thinned with a reducer and a hardener so that creates a bit more liquid. It also gets spread pretty thin over the coats. I would imagine 2 passes of basecoat would do it.

Also, remember, another gallon of clearcoat, also with its reducers/hardeners.

It is never the paint that is expensive with paint jobs. Its the labor due to teardown and basic shop rate. I would say 90% of the cost is always the labor involved.