You shouldn't think of the grammatical verb forms involved as tenses. It's the same as in Russian when you say, as the title of another thread suggests, "Я русский бы выучил...". While the verb form by itself looks like past tense, you are not necessarily talking about the past here. It's a typical feature of many Indo-European languages that they express the idea of "subjunctive", of something to be decided, something not real, something wished for, by shifting the verb back in its tense.

You get three different standard types of if-clauses:

1. If it is not so expensive, we will open a shop.
This type is for a decision to be made. The speaker has not decided yet, he needs to know how much opening a shop will cost before he does so.

2. If it wasn't / weren't so expensive, we would open a shop. In this case the fact is "it is expensive" (now, present tense), and therefore the decision not to open the shop already stands.
You could argue for a variant "if it wasn't / weren't so expensive, we would have opened a shop" in order to express that it was, is and will be expensive in general, and therefore you never opened a shop at any time in the past.

3. If it hadn't been so expensive, we would have opened a shop. This is the more typical way to express the previous thought, back when the decision whether to open a shop had to be made, it was too expensive to do so, so it was decided not to open a shop. This sentence leaves it open whether opening a shop is still expensive now.
I suppose here you could argue that you have the option to say "If it hadn't been so expensive (back when we opened the first shop), we would open another shop now (but it's out of the question because we cannot afford that)".