Sup. Ins

I was wondering if/what did you find out about your mothers supplement insurance because I am doing the same thing.....trying to find out which is best and a reasonable amount of money. At this time my parents have AARP as supplement ins. to pay the 20% medicare don't pay and it cost over 400 a month, that takes both of their ss checks to pay that. It has to be a better way. Any advice is appreciated.