Questions & Answers about Die Kosten betragen nur drei Euro.
Kosten is a plural noun in this sentence, so it takes the plural article die.
- die Kosten = the costs
- Singular forms do exist in some contexts, such as die Kosten being used as a plural-only idea in everyday language, while der Preis is often used when talking about the price of one item.
So die Kosten betragen ... is a normal way to say the costs amount to ... or the cost is ...