I just finished an interior wall. Divided a third bedroom we weren't using into a front space for the kitty litter box and a bit of storage and the rear space will be walk-in closet where the cats are not allowed (they love to sleep on clothing.) Total cost for a ten foot sheetrock wall with a pre-hung door, trim, 2x4's, mud, tape, sheetrock, screws, nails and paint was $608. That include some outside labor. I built the wall, hung the drywall, but hired someone to do the mudding. He also helped with painting to fill out a day's labor ($200). So $200 labor and $408 for parts.
I made it so that it could be removed fairly easily, didn't cut the original moulding, curved the ends of the new moulding to fit over the original. Tearing it out, there would be some spackling and painting, but nothing else.