Prototypes are expensive. Luckily, many new technologies can help us with that. First and foremost i strongly recommend 3D printing.
Watch this example on the incredible power of 3D printing.
3D printers enables you to have a cheap overview of your design and even motion testing.
In other cases, whatever the material you are planning to use, there is a cheaper one, less expansive, less strong and less costly to produce.
in this case we used a less thicker aluminum instead of steel. A lot cheaper to cut and bend.