Learn Portuguese (Portugal): O café custa um euro.

It means both! In Portuguese, the word for the number one (um for masculine words, uma for feminine words) is exactly the same as the word for "a" or "an". In this sentence, because it's a price (um euro), it makes the most sense to translate it as "one".

Why is there an accent mark on café?

The acute accent (´) tells you two things: which syllable to stress, and how to pronounce the vowel. It means you stress the final syllable, and the "e" makes an open "eh" sound, rather than the closed, almost silent "e" sound often found at the end of European Portuguese words.

Is custa a regular verb?

Yes, it comes from the regular -ar verb custar (to cost). Because o café is a singular noun (acting like "it"), we use the third-person singular ending. For regular -ar verbs, you just drop the "r" from the infinitive, which gives us custa.

Why isn't the currency capitalized in um euro?

Just like in English, the names of currencies like euro and dólar are common nouns. They are always written with lowercase letters in Portuguese unless they are the very first word of a sentence.