Ah I didn't see your second post at first, tried to edit ran out of time.. sorry for the multiple posts!

They can survive on rats but it is very messy. I would be hesitant to do this myself for the sanitation issues and unless you breed the rats it would be more expensive than buying food. They need to eat every 4 hours as adults, so they need one rat per ferret per day and you'd have a rat carcass or two sitting in the cage ALL DAY. You would have to keep them separate all the time for this as they can't/won't eat all they need in one sitting and over live/raw food get very possessive/competative with eachother.