I sold my last NPC Proback for a Nikon FM series for something under $200 five years ago but they were all around $600 or more new. If you are commited to 35mm commerical - location work with lighting then they are one of the most valuable things you can have. The images are very sharp, and if you bring a slide loupe with you, you can peer into the Polaroid and get as much information about the composition and lighting as the larger format Polaroids you are used to.
NPC also made these backs for Canon, Leica R, Minolta, and for medium format cameras like the Pentax 67 or Mamiya 6, which didn't have interchanable backs like a Hasselblad. Contax made another version called the Contax Preview.
Nowadays most commercial shooters use digital, or at least use digital as a proofing material. I still shoot large format 4x5 Polaroids for proofing jobs(usually architecture), but usually I wait on shooting 4x5 Polaroids until I have things pretty tight. I use my D70 as a "rough" proofer, just looking at it's tiny display screen is enough to give me the idea early on.
That way, I am only using a couple of Polaroids. Back in the old days, it was nothing to use up a dozen plus Polaroids getting ready for a complicated shot.
Newton was probably using a Polaroid 110 camera, which were often converted to use the modern pack films for medium format. Polaroid and Konica also make (made...) contemporary cameras that used the larger medium format film. The advantage is a bigger print, and they are useful if you use an odd-ball camera that won't take a Polaroid back. You loose the advantage of seeing the scene through the same lens from which the final photo will be made, but at least you can tell how the lighting is. People also used these on movie sets, before video was available to check the lighting. They are nice cameras to have, although they won't do as many things as the dedicated 35mm NPC backs.