Keep in mind that Black Friday and Cyber Monday just happened.

Helium

It’s worth learning from an organization that does the work of preservation of paper on a truly massive scale:

https://siarchives.si.edu/what-we-do/preservation

“The 4RU chassis can be fitted with 106 3.5-inch hard drives…”

106 x 29 = 3074 terabytes

2.5 petabytes = 2500 terabytes

3074 - 2500 = 574 terabytes

574 / 29 = 19.79 so about 19 or 20 drives out of the total 106 used for (parity? hot swap?).

I have a feeling that this might be slightly out of my budget.

Online, you aren’t getting 6TB+ for under $100 without going used or very sketchy. The best I see at a reputable vendor like B&H Photo is a new 6TB external WD for a Black Friday sale price of $130. Just look at https://shucks.top/, what you want isn’t happening.

Watch your local Costco, Best Buy, Walmart stores - you might get lucky if they put out a palette of externals for a special price.