If you want to do a color change without it looking stupid when you open the doors, they have to spray the inside door edges and door jambs and stuff, which they don't normally do. That's where the extra expense comes in. Not only spraying the paint, but the extra sanding and masking too. But if you don't mind the door jambs being black still, then it's no problem at all.
Most places will use a primer sealer (maybe not for a Maaco/cheapo job. Ask to be sure), so there is no chance of it bleeding through, and coverage (black showing through) is not a problem either. The primer sealer (as opposed to primer filler, which can be gray, red and so forth) is a medium gray.
ASE Certified Tech
Used to work in Auto Body and still do all my own stuff.