I need some source (preferably free) for real-time (or some reasonable time lag) for quotes of cryptocurrencies. Can you suggest anything?
What does Kraken or Poloniex use?
the price on an exchange will be determined by the user. the user (=trader) decide for how much {money, other coins, whatever} they buy or sell a coin/token.
if you want to have the prices of many exchanges/many coins/many token you can use the coinmarketcap api for example. coinmarketcap calculates the average price of several well known exchanges. but in practice every exchange has an api which provides the data of the current orders/price. see for example api-docs: gdax, poloniex, binance, bitfinex, bittrex, kraken.
