The price of cryptocurrencies is determined by supply and demand, just like other commodities. Many alt coins are indeed scams or ponzis. Bitcoin has been demonstrated not to be a ponzi scheme in the classical sense, although it is certainly possible that they end up worthless in the long term anyway.
I have the same feeling that Bitcoin in some days will cost nothing at all. BTC does not secured and as an every financial instrument in the wrong hands it may burst. For example, the scheme of mortgage securitization was great but in the hands of greedy bankers it resulted in a crisis