Re-read the English you wrote Tomii: its quite unnatural to begin with. Would you really say 'I'm going to take 5 kilos of them'? First of all, 'I'm going to take' might not even be grammatically correct in this sentence because it implies decisive action, whereas what's really happening here is an indirect request. And why add 'of them' in English? It would be quite redundant.
'I'll take 5 kilos.' would be a natural way to say this in English (though obviously not in a conversation happening in America).
Also, 'What's the total cost' and 'It's 10 dollars' both sound highly unnatural in English. They sound like sentences from some of the textbooks my students are using right now over here in Japan. If you are going to translate from English directly into Japanese (which isn't ideal anyway), at least use natural sounding English as your source text.