1) As a few of the others point out, "priciest", "costliest", and "dearest" (highly regional in the sense of "most expensive") are good antonyms of "cheapest".
2) As einpoklum points out, however, "cheapest" would not be a typical adjective in this construction. ("He sells them cheap" is rather familiar for what could also be rendered "He sells them cheaply", "at a cheap price", or what have you.) The likelihood of finding another adjective that can be used in this unusual way is low, which is why some people are proposing things like "best" and "most". Those work syntactically but unfortunately have nothing to do with price without a few extra words of context.
3) As paxdiablo points out, by far the most important observation to be made here is that there is no reason whatsoever to translate one word in the source language to one word in the target language. You are guaranteed to produce a bad translation if you follow such a rule dogmatically, because languages are not 1:1 codes of each other's words!
As an occasional translator, I recommend going for a paraphrase — even if it ends up being a couple more words than you expect — rather than trying to cram the meaning into one word.
My suggestion: "...the prettiest package sells for the most" or "...for the highest price". An option for an idiomatic expression — in fact, probably the most apt cliché in a sentence like this — is "fetches": "...the prettiest package fetches the highest price."
Alternatively, as has also been suggested, if you take away the verb "sells" in favour of the copula "is", you can use an adjective like "priciest" or "costliest" (although in my estimation neither term's nuance is quite right for the passage). This of course requires breaking the one-word rule anyway and changing the structure of the sentence, so I don't see much of an advantage.
Incidentally, use "who is unanimously accepted" instead of "that is unanimously accepted", or simply delete "that is".
Edit: And see Mari-Lou A's note below as long as we're proofreading translations! To resolve "success" model, I assume you want "successful model", but without seeing the source it's hard to know exactly if that's what was meant.