“Cheaper prices” is an error. You may have seen such a phrase many times. “The prices in this shop this week are cheap” – this is simply wrong. Or “cheaper prices on Amazon” this week. Wrong and confusing.
Why? Unthinking minds abound (las mentes irreflexivas abundan). Think about the phrase. What is cheaper? The products or services, of course!
Have you ever bought a “cheap price”. How does a mortal buy a cheap price? I have not, at least on this planet. No, you have bought a cheap product.
Products and services can be cheaper in one business or in another shop. Here the word “cheaper” is giving information about the products or services relative to the prices in another place.
A similar error occurs with “expensive”: the prices in that shop are expensive. This is simply wrong, and not a mistake you should make in the Cambridge English exams. A price cannot be cheap or expensive: it is the product or service that is considered cheap or expensive?
You could say that the Porsches in the BBB showroom in Tenerife are expensive – here we refer to the cars. If a mortal says “the prices are high in that shop, he may mean that “the prices of the Porsches are high in that shop”. If the context is clear, then, perhaps, there is not a problem.
However, to preserve your correct grammar, and not to confound (confundir) mortals, remember the golden rule ABC (accuracy, brevity, clarity = la exactitud, la brevedad, la claridad).