Well if you invent it, it will get copied and churned out at a fraction of the cost. And so GoPro cages are turning up on Aliexpress dirt cheap.
A simple and quick search reveals multiple GoPro cages selling from $35 up to $70+ and most have free shipping on Aliexpress.
Gopro Cage New Gopro Accessories Gocage for Gopro HD Hero2 and Hero3 CNC Made Golden Color
New Black CNC Aluminium Cage GoPro HD Hero 3 Camera protector
Free shipping GoPro HD Hero3 Camera CNC Aluminium Cage, black, gopro accessories
Aluminum Metal Skeleton Rugged Cage Protective Case+Lens Cap for GoPro Hero 3+
Second generation aluminum alloy shell multifunctional dog cages dog 3 metal shell gopro hero3
This was just a simple search… The GoPro cages come in various colours and accessory combinations.
There are probably even more cost effective GoPro cages out there.