Not to mention there are some with like 128 or 96 GB of ram and SSD-s also inside..
That's not how it works though most time. When a whole node-pack is listed in this fashion, you need to divide it per node, and on average the nodes come with 24GB, or 48 in more expensive variants.
Do your calculations precisely though, because from what I observed, you often get only notch higher performance compared to conventional contemporary builds. After all, you are buying core-2 generations xeons or early westmere ones. The higher-specced C6100 often go up to 4000 dollars on Ebay (8x X5670, which actually is quite serious performance) but even than you save about 40perc. of price (ok, that can be a lot to some) to performance of contemporary custom build.
It looks like amazing budget deal if you only count "cores" on paper, but if you take into account their architecture family, frequency, and power draw...well, it's not that amazing anymore. It has quite too many drawbacks imho.
Not that it can't be good deal, but I see a lot of stary-eyed guys on internet forums thinking like it's amazing deal. Then I look at it...and, not really. Be cautious.