That's typical with a lot of plate, bearings, ect... It all comes down to order volume and standard parts. Example, electrical panels with knock outs punched in are cheaper than an identical panel without knock outs even though there is less labor in the part. Because people don't often order panels without knockouts so they don't produce as many.
As for getting the plate, look for a local metal supplier and ask if they sell drop pieces. It may not be the exact size you need, but chances are they will have something close enough and you can get it at or close to scrap price. They will often have material that is too cosmetically scratched to sell as well.
