There's a really simple explanation. The cost per coin could easily head north of $300 again when demand out weighs the supply. If there is enough adoption 21 million coins is not enough to go around. Also the currency velocity is a dull argument, bitcoin's only purpose is not for transactions but also a store of wealth and possibly even a reserve currency one day. We are still in a high inflationary area of the new bitcoins per day, but needless to say this drastically decreases over time. It's a digital asset that cannot be faked. If 21 million people were interested in bitcoin the price would easily head north of 4 figures.  There's a lot more to the bitcoin price than just mining costs.