Perhaps one of the biggest reason why the Nitto (and Thomson) seatposts are so expensive is where they are made. Japan (and the US of A) do not exactly have cheap labor. On top of that, the manufacturing conditions (e.g., environmental restrictions, health regulations, etc) add quite a bit to the cost. Due to the inherent high baseline cost, it makes no sense to use cheap material/processes, so you'll also getting one of the best material and manufacturing processes (including QC). It all adds up.

The cheaper seatposts may be just as strong, but they may not be as consistent and you may loose out a bit (or a lot) on making sure the labor is fairly compensated and adequate care had been taken to mitigate environmental impact. To make the seatposts that much cheaper, some corners have to be cut. These corners may have nothing to do with the functionality of the product, but corners were cut nevertheless. It's not as if anyone's getting rich making seatposts or any other bike parts!

We all make our choices, hopefully with eyes wide open.