The unfortunate answer is that once you invest in a specific razor, like one made by Gillette, you are at the mercy of its pricing demands.
Since each individual razor will only be compatible with the brand’s own blades, the company can charge the consumer as they see fit for replacements.
While this isn’t an ideal arrangement for the consumer, if you are loyal to the brand and enjoy their product that much more than the competition, then it’s a small price to pay.