Learn French: La transformation qui plaît aux habitants coûte cher. - The transformation that appeals to the inhabitants is expensive.

It acts as a concise replacement for a verbal clause like le fait de transformer (the act of transforming) or ce qu'on a transformé (what was transformed). Nominalization allows you to express complex actions as simple noun subjects, which is very common in B2-level written French.

Since la transformation is feminine, why isn't it coûte chère?

In this sentence, cher acts as an adverb modifying the verb coûter (to cost), rather than an adjective modifying the noun transformation. When cher is used with verbs like coûter, payer, or vendre, it is invariable and always stays in its masculine singular form.

Why is it aux habitants instead of just les habitants?

The verb plaire (to please or to appeal to) must be followed by the preposition à when indicating who is pleased (plaire à quelqu'un). Here, à merges with the plural article les to form the mandatory contraction aux.

Why do we use the relative pronoun qui here instead of que?