Does it need to be in metal for a prototype? If not, do a 3d printed model and, if needed, wrap it in metal sheet/foil.

There is no cheap way to do a production quality prototype.

Besides 3d printing, the only "cheap" way is to join a tech shop and make them yourself.

Any craftsman will probably charge in the thousands. That being said you should call around to fabrication shops and find out their prices.