They won't save that much in material by buying bulk.

People think that just because it's a flat bit plate, that it's easy to machine, but it's rarely the case. Taking your end plates for example, how would you hold them down?
On a router table, you could just screw the plate to the bed, or rely on a vacuum table, but I'd doubt they'd be using a router to machine them.
On a mill, you could clamp to the bed, but then you have to work around the clamps. You could use some form of low profile vice fixture, but you're still looking at two setups. Other option is to make a fixture plate, but that's additional cost and machine time.
Even your small plates, if you've specified tight tolerances around the full perimeter, instead of just using a bit suitably wide bar in a vice, milling both ends flat, maybe surfacing the top face, and drilling/tapping the holes, you now need to have two setups so all edges can be machined. Every time you have to do a setup, is time that has to be paid for by someone.

Plus you've specified a blasted finish, so that's more cost.

The quotes do seem high, but when you actually breakdown the time involved, they're probably not that high for a professional machine shop doing a one off job.