Which is correct: "값이 내린다" or "값을 내린다"?

Both 값이 내린다 and 값을 내린다 are correct because 내리다 can function as both an intransitive and a transitive verb.

• 값이 내린다 (intransitive)
  Prices are falling.
• 그 가게에서 값을 내린다. (transitive)
  That store is lowering prices.

Notice that the intransitive 내리다 can be translated as "to fall," and the transitive 내리다 can be translated as "to lower." Since 내리다 can function as both an intransitive and a transitive verb, there is no real need for a passive form. For example, there is no need to say 값이 내려진다 (Prices are being lowered) since 값이 내린다 (Prices are falling) essentially conveys the same meaning. Besides, 값이 내린다 is more Koreanese.