The point of the Bancor was to have fixed exchange rates but also deal with the trouble of trade imbalances, which are a problem on a fixed exchange rate. On a fixed exchange rate, trade deficits are a problem because it means you are running out of the reserve currency, so you need a trade surplus to supply them. But the burden falls on the deficit country (who will endanger their peg if they run out of reserves) and not on the surplus country (who is happily accumulating foreign reserves). Under the Bancor plan, the negative interest rate on holdings of Bancor would have encouraged surplus countries to spend down their holdings, moving towards trade balance.
The way to think of pricing cryptocurrency is as speculation above some floor determined by the growth of the network according to Metcalfe's law. There are also various other metrics that probably correspond to price like the number of transactions, the number of wallets, and the hash power wielded by the network.
Some exchanges face banking issues and so can't hold fiat moneys. Instead they turn to a centralized cryptocurrency called Tether which is pegged to the price of the dollar. There are claims that this currency is not backed by real USD and that it is being used to manipulate cryptocurrency markets.