Learn Icelandic: Er stóllinn ekta? Þessi sjaldgæfi kostar mikið. - Is the chair genuine? This rare one costs a lot.

In Icelandic, you don't need a word like "one" to stand in for a dropped noun. Instead, you just use the demonstrative pronoun (þessi) and the adjective (sjaldgæfi) together. Because they are in the masculine form, they clearly point right back to the masculine noun stóll (chair) from the previous sentence.

Why does the adjective sjaldgæfur (rare) change to sjaldgæfi here?

Words like þessi (this) force the following adjective into its "weak" declension. For a masculine, nominative singular adjective, the weak ending is -i, making it sjaldgæfi.

Why doesn't the adjective ekta (genuine) change its ending to match the masculine noun stóllinn?

Ekta is one of a small group of "indeclinable" adjectives in Icelandic. This means it never changes its form, no matter the gender, number, or case of the noun it describes.

Why use the neuter form mikið (much/a lot) at the end of the sentence?

Here, mikið is acting as an adverb describing how much the chair costs. When an adjective is used as an adverb to describe a verb, it takes its neuter singular form.