Extra insurance elderly parents?

I have Medicare, but Part D (for prescriptions) through a private company which is approved by Medicare. It charges me nothing per month, nothing for my one prescription; however, if they take a lot of medication, it could be worth it. If you find out how much their medications cost a month and compare it to the $500.00 they are paying out for extra insurance, then you can tell if it is worth having it. I do not know what IRBA is.