So about $800 in HDDs. Steep but reasonable for all the knowledge man has acquired so far.
Unlike wikipedia, they don't offer a full torrent of the site, so it's hard to estimate. I guess the closest point of comparison would be Anna's archive index, since they have a rough filesize total?
I was going to comment that Library Genesis went offline earlier this year, but I'm very happy to see that I'm wrong and it seems to be back.