"It is not just cheap."
Translation:Non è soltanto economico.
Tread carefully. Google translate might not always explain everything ;)
If the sentence were rearranged you could very well use proprio, but the current word order indicates that the subject (it) has more than one great feature aside from being cheap, it is not only cheap. It is just not cheap or It is simply not cheap would be a more appropriate use for proprio = just.
I think the high level lessons should have deeper explanation for each word.