If it were only true! Your arithmetic is good but the raw data unfortunately is a bit overoptimistic. I would LOVE to duplicate my 8 big honking golf cart batteries with only 500 or so cheap lithium cells but it won't happen. Most of those cheap cells with wild capacity claims are actually only about 1200 to 1800mah and some are even less. I have heard of them testing out at 900mah.
Possibly the most costly element is not the cells themselves, but the BMS required to keep so many cells playing nicely with each other at a high degree of safety and reliability. Better cells will spec out more consistently, Careful testing and matching will help. Panasonic is probably the best brand and they will not be cheap. There is so much difference between good (Panasonic) and bad (brands you haven't heard of, from China) that there are probably counterfeit Panasonics being sold.
DIY building a 18650 bank or 26650, 32600, 32650 or whatever size cell bank is much more practical where thermal runaway is not such a big deal. An ebike pack, for instance. Otherwise you gain very little by not just buying a balanced pack from a manufacturer or salvaging from an electric car. Far simpler to go with large prismatic cells, big enough that there is no need to wire in parallel, I think. The BMS can be quite simple or possibly not even used, with space for air circulation and individual fuses between cells, I am thinking. But for now I am sticking with my golf cart batteries.
Keeping my eye on the Samsung 21700 though. Tesla will probably be making kajillions of them under license, soon.
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