I'm not surprised if their actual costs are in the range of thousands of dollars, because they typically have horrible typesetting workflows that take your nice LaTeX, mangle it into QuarkXPress and give you a PDF for proofreading with tens or hundreds of annoying small errors. Which you have to find and point out, and then they fix every single one manually in their horrible software.

It's not the cost of a value-adding service, but the cost of having a huge technical debt that nobody has bothered paying down on.