I had the poster sent out for conservation: de-acidify and mounting on linen (but no bleaching.) Cost: $180US. At the same time I wanted to have the poster scanned, as it was my intention even then to try and sell a few copies to defray the expense. You would think the firms specializing in movie poster conservation would have a large format scanner and do this kind of copying and reprinting all the time, but no. None of the better known and reputable firms in the trade had this service available. I had to settle on a 4x5" photographic color transparency which they provided for an additional $25US. This I them scanned in at 2400 optical dpi on a typical desktop flatbed scanner and then enlarged, cleaned, and tweaked in Adobe Photoshop. But not too much, I preserved the slightly crooked printing and some of the ink blobs of the original!
Though the 4x5" transparence was far better than a 35mm negative or slide would have been, the edges of the text were still fuzzy and needed sharpening up. Also the scanner captured the millions of color variations and hues not desired in what is supposed to be a two-color poster. It was quite a bit of work to clean up, even with the tools and power of the image editing software. I was fortunate to have access to the software and hardware. The same goes for being able to print the tabloid size, for the cost of the photo paper only. I also have access to a large format printer that would do the poster full size. But that particular device was only capable of 300dpi, and the results were not up to my standards. So I have withdrawn the offer of full size copies for now. I am trying to convince the owner he needs to upgrade to a better large format printer!
There may be blueprint coping services that do have the large format inkjets that will do 600dpi or better (and the architects use color). Certainly they have the scanners, but what the resolution of those is I am not sure. I only ever used them for photocopies of engineering prints which can get by with only 150dpi; and my local one could not handle my electronic image files. The cost was likely to be nearly what I felt I could sell the poster for, with yet packing and post to pay. But you may want to investigate that line and see if it is more reasonable in cost down your way.
So that is how the price is so low, I do most of the work myself for five cents and hour wages, on borrowed equipment!
Recently I have been working on some 1913 atlas plates about 22x34". These I have being scanning in 1:1 in 8x14" chunks with a HP4670 scanner. You lay the work out flat (nice for large originals) and place the scanner face down on top of it. The scanner is different from your average desktop unit being a 5/8" thick see-through panel completely flush on the bottom surface. Then stitching of the mosaic together and doing the cleanup work in Photoshop. The details are sharper, given the 1:1 scan. If I were I to do the poster now I would go this route. It takes a very long time for the very detailed atlas plates, but a poster being simpler would not be so bad. I have done the same with regular desktop scanners, but some object nosily to being inverted and may jam, and the bezel around the edge lifts the paper away from the glass causing parallax distortion that needs to be digitally trimmed away and discarded.
-Doug