Imagine you have a customer who wants to scan 250,000 slides, negatives and prints... in good quality. If you manage to make a good 35mm slide scan with, let's say, a Nikon 9000, within two minutes, all work included, the scanner will run 24 hours for one year.
I know that some people use offshore scanning services for large volume scanning. I heard some companies in India are specialized in this area of business. Can you recommend some of them?
Depends what you want. A scan is not just a scan. There are a lot of choices one can make. They use ok scanners... not great ones. And they just scan what you give them. There is no concern over what the image is. If you don't care, all you want is a catalog of things, then fine. If you are fine with prints from Costco, then it will be fine for you.
The folks on the high end scanner list all agree - a lot depends on the operator. One can get lucky, but if you want consistent results, you have to have a human. Just like its the human, not the camera that makes the image, it is the same with a scanner. It takes some intelligence (or awareness of context) to run it.