That's still close to the cost of the housing. Dome ports shouldn't cost THAT much, and they wouldn't if there were a standard dome port that every photographer used. Edmunds Optics sells mass-produced 8" hemispheres for about $50, the mounting features can't cost THAT much.
If, as a Manufacturer I can make a product for a theoretical price of $100.00, by the time I add Gov Taxes and duties, Transport costs, material costs and a reasonable profit for my self and then the wholesaler gets his profit and then the retailer adds his the price for that $100.00 article will now be about $400.00 - $500.00 dollars. Now take into account the cost of the machinery to manufacture this item and you start to ask yourself as a business why am I doing this!
The Dome from Edmunds are far to thin material (1/8") for an optical dome and they are not optically produced or coated.
This is not a cheap sport and U/Water photography is not a cheap pastime!
Peter