Learn Italian: In questa città, la comodità costa tanto. - In this city, convenience costs a lot.

It comes from the adjective comodo (comfortable or convenient). By dropping the final vowel and adding the suffix -ità, it turns into an abstract noun. This is very similar to the English suffix "-ity" (like "validity") or "-ience" (like "convenience").

Why does comodità have an accent mark at the end, and what is its plural form?

Abstract nouns formed with the -ità suffix always end with an accented -à. In Italian, nouns that end in an accented vowel are invariable, meaning their spelling doesn't change in the plural. To say "the conveniences", you just change the article: le comodità. You can see this same rule applied to città (city) earlier in the sentence!

Why is there a la before comodità when the English translation just says "convenience"?

In Italian, you must use a definite article (like il or la) when talking about an abstract concept in a broad, general sense. Since comodità is feminine, it takes la.

Can I use molto instead of tanto here?

Yes, absolutely. In this sentence, tanto is acting as an adverb to describe the verb costa (costs). When used as adverbs meaning "a lot," tanto and molto are completely interchangeable.