The tense you're using in your examples is not the imperfect, but the pluperfect--the imperfect subjunctive for comprar and ahorrar in the first person singular are comprara or comprase and ahorrara or ahorrase respectively. The question holds anyway.
One of the most common uses of the subjunctive mood is in conditional sentences, but it's not the only one. You can use the subjunctive in sentences expressing doubt, for example. This is independent from the tense. You'll be learning these uses in time. There are a lot of examples using conditional sentences in the textbooks because conditionals are used very often and they are not difficult to learn as a pattern.
An example of non-conditional pluperfect subjunctive:
No creí que hubiera hecho eso.
"I didn't believe that he had done that."
This other sentence is another matter, though:
Me hubiera dado vergüenza de copiar de alguien en clase.
"I would have been ashamed to copy someone in class."
Here the "proper" sentence would be
Me habría dado vergüenza de copiar de alguien en clase.
using the conditional mood. But in Spanish, historically and still to this day, the subjunctive and the conditional have often been confused, each one taking over functions from the other or being used one instead of the other. So it's very common to hear me hubiera dado vergüenza instead of me habría dado vergüenza, and it's been accepted by the language authorities and nobody will find it wrong.
(Outside formal speech it's not rare to find the opposite, i.e. the conditional used instead of the subjunctive; but this is definitely wrong and strongly discouraged.)
Note also that the sentence is in a way a conditional statement:
Me habría/hubiera dado vergüenza de copiar de alguien en clase.
actually means you would have been ashamed if you had copied from someone. The conditional or subjunctive mood conveys this hypothetical feeling. You could express it like this:
Me habría/hubiera dado vergüenza si hubiese copiado de alguien en clase.
(Hubiera and hubiese are exactly the same; they just happened to come out that way but you could use the same one in both parts of the sentence, or invert what I did.)