Learn Hardcore Korean: pati biyongi eolmana deulkkayo? - I wonder how much the party will cost.

이 is the subject marker in Korean (used after nouns ending in a consonant), showing that 비용 (“cost”) is the subject of the verb 들다 (“to cost”). If you used 은/는, you’d be marking 비용 as the topic, which changes the nuance:

비용이 얼마나 들까요? “I wonder how much the cost will be.” (purely asking about the cost)
비용은 얼마나 들까요? “As for the cost, I wonder how much it will be.” (implies some contrast or background context)

What does the verb 들다 mean here?

들다 literally means “to enter” or “to lift,” but when paired with 비용, it idiomatically means “to cost.” So 비용이 들다 = “a cost comes (into existence)” → “it costs.”

How does 얼마나 function in this sentence?

얼마나 is an interrogative adverb meaning “how much” or “to what extent.” It modifies the verb 들까요, asking about the magnitude of the cost. In general:

Before a noun: use 얼마 (e.g. 얼마 돈이 있어요? not common)
Before a verb/adjective: use 얼마나 (e.g. 얼마나 비쌀까요?, 얼마나 들까요?)

What is the role of -ㄹ까요 in 들까요?

-ㄹ까요 tacks onto verb stems to express the speaker’s wonder, suggestion or invitation, similar to “I wonder…,” “Shall we…,” or “Will it…?” in English. Here it softens the question into “I wonder how much it will cost.” It’s polite and indirect.

Is 비용이 얼마나 들까요? formal or informal?

This is polite (높임말). The verb ending -ㄹ까요? sits above the casual level but below very formal speech. You’d use it with people you’re not extremely close to—colleagues, acquaintances, service staff, etc.

Could you rephrase it to sound more casual?

Yes. In casual (반말), you can say:
“파티 비용이 얼마 들어?”
Dropping -요 and shifting -ㄹ까요 to plain -어? makes it informal.

Can I replace 비용 with 돈 here?

Absolutely. 돈 (“money”) is more everyday. You could say:
“파티에 돈이 얼마나 들까요?” or casually, “파티에 돈이 얼마 들어?” Both mean “I wonder how much money the party will cost.”