they are very popular in Japan. I wouldn't say they cost "a fortune"; they're a bit pricey, but there are certainly far more expensive bags out there, and Yoshida have a lot of lines at various price points.
I love mine. all the materials used (including zippers and strap hardware) seem to be of a high quality, and the interior pockets and compartments are well thought-out.
also, not all "nylon" is equal.