The cheapest way is to keep your keys on paper. Just write them down and keep them somewhere safe. Extra funds required: $0 (Since you probably have a pen and paper lying around somewhere) Just make sure you don't keep it somewhere it could get damaged or stolen easily. For example your physical wallet would be a poor option. You can keep it in a small safe box if you want. Also consider encrypting your keys so you don't lose it all even if it gets stolen.

Personally the keys I keep on my PC have lower amounts of balance but I use cold storage for my HODLing coins. And I have several copies in case it gets damaged or lost.