The reason it's not silly/wrong is that you can say, and people often do, that something is "much cheaper" than something else, and that clearly means that it is less expensive. So, the language already accepts that a thing becomes less expensive as it becomes cheaper.

So, if it becomes twice as cheap, it becomes half as expensive. Similarly, as something becomes less cheap, it becomes more expensive, so you can also say that something is half as cheap as something else, so it's twice as expensive. There is no ambiguity, it's just an uncommon way of quantifying price changes. Without the speicifc quantifier, the relative comparisons are actually quite common - it would be cheaper to buy X; is not as expensive as Y - common phrases.