The best way to work out hypothetical conditionals is to first think of what you want to say as a simple conditional:
If there is ample affordable housing, the rent will be much lower.
To convert it to a hypothetical conditional, You convert the verbs to their subjunctive form. The be-verb has a special subjunctive were: for all other verbs, you backshift (the same as you do for reported speech). Taking the sentence above, you get:
If there were ample affordable housing, the rent would be much lower.
Your suggested sentence is incorrect: was should be were- though you will hear was in informal, spoken English.
Also, it probably doesn't mean what you want to say: would have been relates to a situation that happened in the past, rather than to the present.
For example, if a friend tells you that their car broke down yesterday, you could say:
If I had been there, I would have been able to fix it for you.
As I understand it, a mixed hypothetical conditional is one where the two clauses relate to different times.
First, let's look at how we convert a simple (non-mixed) sentence to a simple conditional. We do it by adding If and a consequence-word, for example will or can.This sentence has two present tense clauses:
I have money, I feel happy.
If I have money, I will feel happy.
Now start with a mixed simple sentence, with a past simple clause and a present simple clause:
I invested a thousand back then, I have nearly thirty thousand now.
We convert it to a mixed simple conditional by adding if and will. Mixed simple conditionals are grammatical but don't always make sense, as with this one:
If I invested a thousand back then, I will have nearly thirty thousand now.
Now convert it to a mixed hypothetical conditional by adding a backshift:
If I had invested a thousand back then, I would have nearly thirty thousand now.