They can both be used. 1. El precio fue barato.(The price was cheap when I went, and now it is not) CORRECT 2. El precio era barato.(The price was cheap when I went, and I don't know if it is still cheap or not for now) Not quite I think for the imperfect it's not really that you don't know, it's simply that you don't take a position on it at all. I would change it to (The price was cheap when I went, and it may or may not still be cheap) or simply (The price was cheap when I went). There is no extra information contained in the statement. Preterite = action definitely complete Imperfect = action is incomplete or it is unknown if action is complete or not. So back to your example, I will add a little context to show, what I think would be a natural usage of each one. El jueves el precio fue barato, pero tras la catástrofe, ha subido bastante. On Thursday the price was cheap, but after the catastrophe it has risen substantially. Clearly, the price is no longer low. Cuando era niño, el precio de gasolina era barato. When I was a kid, the price of gasoline was cheap. The time period of when you are a kid is not concrete or certain, so the action is imperfect due to this uncertainty. Ayer fui al supermercado y, para mi sorpresa, los precios eran muy baratos. Yesterday I went to the supermarket and, to my surprise, the prices were very cheap. The prices were cheap. They may or may not still be cheap.

I would like to add, here, in Spain we tend to use imperfect in a conversation like your example. To say the price is not a new action in the past but extra information about what you did, you describe the price of the fruits, in other words you are giving extra information: A:Ayer fui al supermercado y compré fruta B: ¿Qué precio tenía? ¿A cómo estaba? (extra information talking about the fruit) B: ¿A cuánto la compraste? (new action finished in the past) I hope this helps you to understand.

There is another dimension to your question, and that relates to the use of "ser" or "estar." In the example you are presenting. The answer would depend also on whether you are conveying a message for a particular or a general situation: Los precios en París son altos. Normalmente los precios de los alimentos son más económicos en Madrid que en París. Ayer fui al mercado y las fresas estaban en muy buen precio. En el supermercado de mi esquina las fresas son muy costosas, nunca las compro ahí.

"era" is the verb "SER" in "pretérito imperfecto" "fue" is the verb "SER" in "pretérito perfecto simple" We use "pretérito imperfecto" when we want to express past actions that have had a beginning and an end not specified. It also serves to make clear that an action begun in the past but still has regularity or continuity. We use "pretérito perfecto simple o indefinido" to express actions that happened in the past in a timely manner and have ended or stopped. In your example: El precio era barato. You are saying that the price was cheap and maybe is sill being cheap, you don't know when the price will stop being cheap. El precio fue barato. You are saying that the price was cheap at the time you were there, maybe because a promotion, but it is not anymore. I hope you find the information useful... https://teach.italki.com/teacher/6042164

I would use "el precio estaba barato"