What's "fair value" for a cryptocurrency?

Hi Steemers,

I'm completely new to Steem and fairly new to cryptocurrencies so I was hoping to draw on your vast knowledge to answer the question what fair value is for a cryptocurrency and whether there are any inherent differences in determining the value of a cryptocurrency compared to a regular currency such as USD.

The reason I'm asking is that with coins like Ethereum at the current levels, some people seem to believe that there is still a lot of upward potential for its price purely based on the fact that we may see greater adoption of Ethereum in the future. While I am not concerned with the assumption that the value of blockchain technology and individual currencies like Ether or Bitcoin is tremendous and that we have seen only an infinitesimal fraction of its future adoption, I am concerned with the assumption that this will translate into higher prices as adoption increases. The reason for this concern is simply explained by the fact that the exchange rate of currencies is normally driven by supply and demand which is in the long-term likely to be determined by real supply and demand rather than speculation. Isn't it likely that if price for ETH or BTC increases we will hit a ceiling (may have already hit it) where it will no longer be attractive for those interested in real smart contracts to use these currencies.

I'd really be interested in your take on this and in whether I am missing something here. Any resources or reading material would also be much appreciated!