The cost can depend on many factors because there are many options when digitizing a printed book - kind of like saying what does a Honda Civic cost - it depends on the options you choose for the car. It might be best to start with asking what format you want the final work to be in. Reproducing an illustrated book as a visual PDF is much easier than taking a Calculus text book and converting it to ePub format. There are also differences in whether you are digitizing to create a file usable for Print-on-Demand or ebook or both. Image Resolution - If you have images in your books, then the resolution (dots per inch, dpi) of the scanning done to digitze the work becomes important. If you are only planning to do electronic formats (epub or PDF) with the resulting digitized file then 300 dpi is probably sufficient. Most POD printed books also use 300 dpi. If you want archival quality images, images that can be zoomed, you might want to go with 1,200 dpi or even higher. Higher dpi means more scanning and processing time and thus more cost. Greyscale - Mostly related to images again, older scanning equipment does not pick out as many shades of grey as newer scanning equipment. B&W illustrations on older scanners will come out starker black, with lots of contrast and loss of greyscale detail. Optical Character Recognition (OCR) or not - Most use cases will mean you want to OCR the text when digitizing, not merely get the book scanned and visually reproduced with image files placed in a PDF. OCR means you get the text and the digital file understands it as text, not just as an image of a page that happens to have text on it. Edit OCR text or not - Text extracted by OCR usually needs to be copy-edited as no OCR process is perfect. The quality of the editing work now becomes a subjective factor. Is it just run through automated spell correction? Is it edited by someone fluent in the books' language, etc. Tag it - Do you want extracted and edited text to be tagged for headers and otherwise formatted into a proper ePub file? I know I'm not giving you any definitive answer here, like it should cost $X per page. It really depends on what options you want.
In most cases a publisher would prefer a “publisher branded PDF” (where a publisher logo is superimposed on the image) over a “digital PDF” like one generated by Caliber. When you buy a digital PDF from a publisher it usually can't be Oared. At best, it will be cleaned up to make it readable. That said, there are publishers who do produce OCR and many do charge exorbitant fees for it. In a perfect world, a publisher would give you every incentive to pay them for this service. If you have access to an image processing software company and can do the OCR (or some other task) then you might be able to get something to a price that matches the cost to run Caliber on a desktop. PUB PUB allows the inclusion/exclusion of images from the HTML for readability of the content. Many books do not have.