at a current rate of transaction volume bitcoin needs to cost 15$. Everything that is above that price means that some1 holding bitcoin as investment and may be willing to dump.
Indeed. What you are saying is that bitcoins actual fundamental as a currency must be around that.
However, the actual market price may include speculation on a future, higher fundamental. Nothing stops one from thinking that bitcoin will, one day, be used as a currency so much, that its *has to* cost, say, $100,-, or $500,- or $10 000,-.
The top of the scale is about $ 3 million, when bitcoin would eventually replace all of the world's M2 fiat.
So depending on the probability you assign to that future price, and the time when you think it will occur, the current "true" market price may as well be higher (or lower !).
So if you give it a 1% probability that 15 years from now, the bitcoin price is about $ 100 000 (because it took over 3% of the fiat market), you would be willing to consider about a price today of $ 1000,-, diminished with the normal interest rate over 15 years. So maybe something like $ 600,-.
But you're probably right that the current necessary price for bitcoin as a currency for its use to buy stuff with, must be a 2-digit number or so. Mainly probably on the black market.
