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.
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.