It depends on how "mainstream" bitcoin gets. Lots of commentators use the current total value of various liquid asset markets, such as gold or M1 money supply, as a rough idea of the market cap bitcoin could grow to.
Taking gold as an example, the ten largest central banks hold a total of about 33,000 tonnes of gold, worth about $1.6 trillion. If bitcoin could cut into, say, one quarter of that, it implies a market cap of $400 billion, or a price of about USD 20,000 per BTC. (Of course, there are good arguments to be made that central banks will never use bitcoin as a reserve asset like gold, but this just shows one example of a rough reckoning for bitcoin's value.)
The price forecasts get more zany & bullish the larger the market you look at, and the larger share you assume bitcoin will take. I personally view it as an option on the potential creation of a new monetary order (with all the risks implied in option ownership). The more that governments and banks use their control over money to dictate the financial lives of their denizens and depositors, the better bitcoin will do.