Are bay roan quarter horses usually more expensive and harder to find? And if so why?
Most any horse of color is priced higher than "plain brown wrappers". paints, roans, cremellos, buckskins, palaminos to name a few. Many times they aren't worth the asking price.
People are shallow and chose horses for their fancy colors.
I don't think it's particularly rare. Roan is characterized by solid points, and bay is black points.