I believe that 300$ may not be enough for a professional full custom metal chassis because the manufacturing of this kind of complex metal parts is a iterative process. You start with some technical drawing, then the machine shop expert will suggest modifications to build the part in a reasonable amount of time and money with the tools at hand. You will be lucky to have a working design at second iteration. Prices and complexity are a lot lower if your design is fully or primarily composed of flat parts to be bolted togheter, because these will come out straight from the sheet metal cutting machine, usually with a good tolerance. Look around, and you will find that most prototype/low volume metal cases are built this way.
I guess you don't plan to use a solid copper metal sheet, therefore the size of the part is not the driving cost for your prototype; the machine tooling time is. Again, you may be lucky and find a metal shop that is already mass producing with a tooling compatible with your part. If you aren't in a hurry, they may fit your nesting between two production lots.
Edit: I believe you are thinking a solution like this
http://www.server8.it/index.php?page=home&lang=EN&type=