What did he use to store that 20Tb in the first place? I'm assuming we're talking a large RAID array. I doubt, from the sound of it, that we're talking software RAID. So, at a minimum, it'll cost AT LEAST the same as building that RAID again to have a backup, no matter what the medium.
Yeah, you're throwing 50% of your money away - on nothing but backups of shit you've probably downloaded or taken off discs you own anyway. So now he probably sees quite what that data was "worth" anyway.
To be honest, nowadays, for home use, just build another RAID the same size and mirror the data across.
Oh, and if you're that daft with 20Tb of data that you press the wrong button and wipe out an array that you have recovered several times over, you shouldn't be let near the low-level storage. Use a filesystem, or even just access layer, with some kind of snapshotting / rollback.
Buy a cheap NAS, or just build yourself a new RAID from scratch and use the "old" array as a redundant copy of the data. Keep it powered off and somewhere else except once a month or whatever when you mirror across.
Backup speed will be good (the speed at which you can interconnect the two computers, basically, probably Gigabit Ethernet for the cheapest scenario), restore speed will be the same, media will be cheap, no fancy software or hardware required, you can re-use your old setups and just buy a new one when it starts getting full and your backups are literally working copies with no further action required.
If that turns out to not be good enough for your needs, that's when you can look at tape and other stuff. To be honest, tape is dying. The places I've seen have weaned themselves off them and just replicate to as many places as possible (including an occasional "offline" copy to prevent automated spreading of bad/corrupt data to the backups).