Translation:How much does the kilo of tomatoes cost?
Yes, if we were going to buy tomatoes, in English we would probably say it like that, mijiturka. But, would Spanish speakers ever say "un kilo" when asking the current price, rather than "el kilo"? I would like to be able to ask the question properly. Anyone know the answer?
If you are trying to speak fluent english you should say "How much does a kilo of tomatoes cost?". Sorry, but your sentence would immediately tell everyone that you're learning to speak english. As Dodojarman says, although an english person would more likely use "a kilo", Duolingo is asking you to translate this sentence using "the kilo". Specific scenario might justify this construction but the most usual way to ask the price would be "How much does a kilo of tomatoes cost?".
In English you would simply say: " How much is the kilo of tomatoes"