There's also the issue of how much of a readership a given title might have --- my last freelance project was a translation of _Thomas Harriot's Artis Analyticae Praxis_:
http://www.springer.com/mathematics/...-0-387-49511-8
Despite its importance, it's really only of interest to scholars of mathematics history (how many of them do you know?), so had to be priced at a pricepoint which would allow its limited sales figures to pay out.
William