Actually, we can estimate the value of dollar and other fiat money as well. First of all, the value of currency can be taken as the value of economy of the country that releases this currency. One more thing of how we can estimate their value is to think about how much we can buy for one unit. As bitcoin is the global currency, we would not be able to think about it like value of economy of one country so we would estimate it based on how much we can buy on one BTC