Permanode is a full node with huge storage capacity to store entire network history securely and permanently, full data history before and after snapshot , basically hourly based snapshots of volume. this type of nodes becomes extremely costly as they get older and bigger. For the small network with limited data not very expensive in beginning. If you running your own physical servers then it would not be very costly only one-time server installation costs after then maintenance costs only.
Facebook, amazon, google and other internet giants running such nodes to secure their data permanently and provide services 24/7 without any interruptions.