

Official numbers here https://www.debian.org/mirror/size
About 4.4TB, but that’s all architectures and (I believe?) all distributions (stable, testing…).
If you only want source+all+amd64+arm64, and only want stable, it will be smaller of course.
Not nothing, but at $10/TB or so, it’s not much.
And if you’re following 3-2-1, I’m pretty sure the “1” is already handled for you :)
You’re right, for new drives it looks like a little more with this 20GB retailing for $230, or $11.50/TB.
For refurbished, I recently got a factory renewed 12TB Seagate for $112 ($9.33/TB), but that price is now up to $199 for the same drive (!).