Translation:Is that cheap?
I'm not a native Japanese speaker, but to me, perhaps because あれら exists and is commonly used, あれ "feels" distinctly singular.
I agree with @Monicat77 that the plural should also be accepted, because in some contexts it can sound natural enough to use あれ as a plural, but in my experience, あれら is preferred in most cases.
In my experience, やすい by itself is almost never used to mean "easy"; one would typically use 簡単 (かんたん, "simple") or やさしい. やすい generally means "easy" when it is being used as a verb suffix, e.g. 食べやすい ("easy to eat"), or やりやすい ("easy to do").
If spoken (i.e. without kanji for reference), this sentence will almost always mean "Is that cheap?"
Indeed, as @Monicat77 says, plural should be accepted since nothing the sentence is specifically singular.
That being said, I would understand if someone accepted "is that stuff cheap?" but personally, I think I would be against allowing it. Firstly, "that" can be (implicitly) plural in spoken English without needing the help of other words like "stuff". Secondly, the "that" in "that stuff" is used more like あの～ rather than あれ (although the same could be said about "that one", I feel like it is a more justifiable exception). And finally, "stuff" sounds distinctly casual to me, which doesn't sit with the use of です in this sentence.
Bear in mind that I'm not an expert, so a third or fourth opinion may also be helpful ;)