A quick check at one service which lists such large amounts, you would be looking at almost $20k/year to keep a single offsite copy of that. That is the posted price however, I imagine that is enough that you could shop around and find a deal, but, a deal is still going to be prohibitive for most people.
At 20 TB I would start thinking about one of two things: Tape, and/or git-annex.
Unless prices have changed since I last looked and the scales tipped, tape has the advantage of being cheap. Of course, you will need to test your tapes occasionally and likely want 2 copies just in case, but, at that point you are invested in tape, may as well.
The other possibility is git-annex and lots of drives, but you can mix types. That way you can keep a catalog of your library and information on where it all is, and how many copies of each thing you have.
Of course, any way you slice it, each physical piece of media is something that can fail so you have to occasionally test to ensure redundancy.