Screen protectors are sacrificial elements that mostly provide superficial protection. Any benefit beyond that is mostly the result of chance, and circumstance, not design.
It makes little sense to purchase expensive ones with warranties, no matter how good, when inexpensive examples of similar quality litter Amazon, some even with recognizable brand names to provide assurance to those who need such a thing. There's usually
an entire thread devoted to them for each new iPhone.
As with something like bottled water, there may be instances where a connoisseur maybe able to discern some differences, but at what cost, and for what benefit?
In both examples, their intrinsic value remains almost static, regardless of cost. The $40 piece of thin glass isn't going to be much different than a $4 piece of thin glass, that, like most phone accessories, probably costs a fraction of that to produce. The $40 pays for the convenience, installation, brand name, and support, not all of which is essential to most users for such a product.
Try one of the cheap ones, and if it's not to liking, try another. One could do that multiple times for the cost of a single Belkin.