If it were sold at the commissary for $5, it'd be easier for inmates to afford (and keep in mind, as the article states, that inmates can only spend $320 a month on commissary goods).
The goal of the prison system is to make prisoners as miserable as possible by our current societal standards (I strongly believe that 500 years from now, our present prison system will seem as barbaric as public place executions seem to us now), not to make sure they can have a pleasant time.
I completely agree, and I strongly believe that prisons are unethical and inhumane. In this case, I was referring to the general public perception- I wonder what percentage of Americans would agree with "the prison system is unethical".
The prison commissary system (largely run by Keefe Group) tends to overprice and gouge for the vast majority of items they offer. This is not uncommon.