Sounds right, if you want black doors!

For good quality paint the material and labor difference for white vs. black should be negligible, especially if this is for dozens of interior doors and not just one. If the builder was going to be doing only one door as black, or spraying every finish in the house with the same white paint, or using lower grade paint, then the difference makes more sense. If you already have a contract that didn't provide for a choice of color, then as a change order ("upgrade") you don't have much leverage. However, if it's a must have, then up to $50 per door starts to sound preferable to arranging to do it later after the builder leaves. If you're talking 2, 3, or more dozen doors then what do you have to lose by making a case with the builder for settling on a flat fee, whether $500 or $1000.

Thanks for the feedback! I'm thinking the doors automatically come white with primer which makes me think they would be hard to clean. We'll have about 20 doors, I'll see if it's possible to bring the cost down a little, like maybe from $1000 to $800. Every little bit helps, right?