Costlier cars need to stand out from the ordinary. And to stand out designers come up with fancy curves, shapes etc. From a manufacturing point of view these 'non standard' items result in more cost which the mid-range cars cant afford. For example there is more wastage in cutting a curvy door from the sheet metal as compared to a standard rectangle shaped one.
It is interesting to note that as time goes by, what was fancy once eventually becomes 'standard' and the features get into the mid-range priced cars as well. Auto-shift is probably a good example. Automatic rear trunk opener is another and so on.