Just an FYI, just because it's wrapped in plastic doesn't mean that it is brand new. Personally I would be very cautious of buying bedding from SA or Goodwill unless you don't mind not knowing for certain the item is brand new.
Big Lots does sell their mattresses separately starting price back when I worked for them was around $150 but that was not quite 8 years ago, they are also brand new direct from the manufacturer. They used to be double wrapped in plastic, but I don't know if they still do that and they have changed mattress brands since we parted ways.
Final thought on the subject is remember you spend roughly 1/3 of your day sleeping so spend a little more on it than the cheapest, unless it's for the unwanted guests room.