Has this got two opposing meanings, or is it simple less-specific (or specific in another way) compared to English? We could express the meaning as:
when cheap → buy
If we turn to the English expressions, we find they both include this basic meaning:
counter-factual: when cheap → buy; but alas it's not cheap unfortunately
factual: when cheap → buy; and there's every chance it's actually cheap
Or in other words, the difference between
is cheap and
were cheap is that it's a different mood: indicative vs. subjunctive. You could say the Japanese phrase is agnostic to mood; much like the English
I give you a letter is agnostic to the social status of both speakers -- it could be used between two two equal or between a worker and his superior.
(※※To put it into perspective, see the note below for a few thoughts on this.)
Back to the Japanese phrase, here's an example illustrating the "counter-factual" usage:
I think: (when a bit cheaper → let's buy). But...
If it were cheaper I would consider buying it, but for now I'm contend just keeping my tabs on it.
If you wanted to emphasize it's not cheap, or won't get cheaper, you could also use:
- もうちょっと安くしてくれたら買いますよ。 (here you are polite and leave the possibility open for the seller not to sell it cheaper)
As for your second sentence, let us put it into context:
(昔日のイラク 1980年代) 「ウイスキーも、安ければとにかく買いました。」
Whisky: at any rate, (when cheap → bought).
And whisky too; I bought what was cheap.
But here the writer actually bought it, at least all the whisky that had been cheap. Now consider:
Here the speakers add a conccessive だが or のに because they didn't actually buy it.
Now we can compare the two sentences:
The difference is simply one of tense: In the former expression the decision has been made and you didn't buy it. In the latter sentence, you might as well still be standing inside the shop wondering whether you should buy it or not.
And as for the two English sentences you mentioned, they work the same way:
- I would buy it if it were cheap. [=it's price could suddenly drop and you end up buying it]
- I would have bought it, if it had been cheaper. [=you decided not buy it, and there might not be another chance for buying it anymore]
Something always gets left out. In natural languages, irrelevant or unnecessary information is omitted. You can't always mention every little detail and everything that could be said. There's no point in specifying the position and velocity of every atom. But languages and customs differ in what is or can be left out.
Japanese nouns can be both singular and non-singular (plural). Now this might sound like two different, opposed meanings. But there's a way to resolve this conflict: nouns are simply not marked for number and don't express any amount explicitly.
Some languages have got a dual form. For example, to a Gothic speaker the English pronoun
we would be ambigous and could translate to:
- wit (=the two of us)
- weis (=many of us)
You could say the English
we has got two oppsing meanings. But the better way to think about this is that
we simply isn't as specific and stands for 2+.
As a last example, consider the verb
(to) give. English doesn't put as marcch focus on social relationships grammar-wise, and thus this kind of information often gets omitted. This is the opposite of the plurals, here Japanese is more specific: 差し上げる, 差し上げます, 上げる, 上げます, くれる, やる etc.