Why are high-sampling-rate data acquisition boards so expensive?
I would not call the example a board.. it's a complete product targeted for a very small niche market.

As others have mentioned, a lot of the cost will be to recoup development, but it also has a number of certifications that also did not come cheap. 

I would also assume that someone also figured out that whatever market they were after, presumably academia, was willing to fork over big bucks.