"de" is not the singular of "des", it is a preposition, while "des" is an article (plural of un/une or contraction of de+les).
The construction "kilo de légumes" (kilo of vegetables) is called "noun of noun", where no article is required.
The second noun (légumes/vegetables) gives information on the first noun (kilo), and works like "une feuille de papier" (sheet of paper).
Not good English, I'm afraid. It would either be "it" or "a kilogram of vegetables", not both. And whichever one you use, it comes between "does" and "cost", i.e.: "How much does it cost?", "How much does a kilogram of vegetables cost?" This is in general; in relation to the above translation, of course, you would not use "it" alone, since it does not translate the "kilo of vegetables" part.
"Alludes" not "eludes", but yes. If you were standing in a shop with a kilo of vegetables in your hand and were hoping to unload them in exchange for anything else, you might look about and ask, "What costs one kilo of vegetables?", although I'd probably say, "What can I get for my one kilo of vegetables?"
I hear what you're saying, but if you closely examine the sentence, you will find this construct everywhere in French. It is very different from English. The only problem is when we try to apply our "English sense" to the French. Translating the sentence word-by-word will often result in a confused, unnatural expression. When we're reading or hearing French, we have to hear it in French. I have mentioned elsewhere that this refers to achieving an understanding of the French sentence BEFORE translating it to English. Then when translating the sentence into English, use the best construct in English to convey the same meaning.