This is a unique item that I cannot find from any other vendor, so I can understand the pricing. However, the quality of the construction is questionable.
The mounting plates are only tack welded to the mounting tubes in two places, and can easily be bent up and down. The mounting tube is not an actual tube, but a rolled piece of flat steel with an open seam on the inner edge (not shown in any of the photos). Finally, two of the four plates I ordered were not welded flush with the upper edge of the tube. As a result, there are small gaps between the plate and underside of the installed work surface.
Keep in mind that these plates are designed to keep a mounted work surface from moving laterally, not to bear weight on the mounting plates themselves. That being said, I think they will serve their purpose well.