I'm not sure if I got your answer, are you talking about Spanish or English? It's an English forum, and if you trying to get your Spanish doubts answered, you are in the wrong place, buddy, but as I'm Brazilian and speak a little bit of Spanish, I will try to help you out.
Both sentences are wrong (IN FORMAL LANGUAGE, thank you StoneyB ), nor Prices are down or Prices are up are acceptable in English nor in Spanish.
Talking about prices means it's a recent stuff, prices rise and decrease all time, so it's a recent matter, that's you must write your statement in the present perfect tense when talking about prices, and there is always one auxiliar verb when talking about something growing up, like prices, these are the commom verbs used when talking about prices:
The prices of this restaurant have risen a lot recently;
This store's owner said he will raise the prices;
If this store's owner doesn't decrease these prices, the store
will go bankrupt.
Don't go to that restaurant, its prices have risen absurdly.
Maybe we should decrease our prices in order to get more
People are going to the opposite restaurant due to its price, we must
decrease our prices otherwise, we will lose customers.