the reason many charge high rates for adapter plates is that locating the center point of the input shaft relative to the transmission bellhousing bolt pattern isn't a tape-measure-and-guesswork job - fractions of an inch out, and there can be nasty vibrations that will shake your car to pieces. A proper adapter plate should also have milled holes for locating dowels and threaded bolt holes where the OEM engine block was threaded, etc.I think everybody will be looking for a capable person that can create adapter plates that don't charge $900 for a $25 piece of metal. Even at just under $500, it would be a plus.
Once someone has invested that much time and energy into designing a precision tolerance part, then made the CAD file, cut a slab of aluminum on a waterjet table, then machined the holes and installed the OEM dowels, they want to be sure they make back enough in low volume sales to make it worth their while.
I agree that $900 is on the high side, but $25 is an extreme under-valuation.