For 1 Euro you get around 24 Czech Crowns (CZK). For 1 USD you get around 21 Czech Crowns (CZK).
The prices in Prague are for example: 200 - 400 CZK for lunch, 450 CZK entrance to Prague Castle
Avoid scammers who overcharge you. Do not exchange money from strangers walking in the street.
In the info of the Exchange Office look for Valuty (exchange of paper money) and Kupujeme (we buy).
If you change your mind and have the receipt, you can undo the money exchange within 3 hours.
If you withdraw money from ATM machine, avoid Euronet machines (high fees). Use a machine standing next to a bank.
Below are some of the reliable exchange offices in the city center:
Exchange s.r.o. https://www.exchange.cz/smenarna (Štefánikova 203/23, Praha 5) - Near Metro Anděl. This is probably the most reliable in Prague. Used by the locals.
Others recommended in the Honest Guide reviews:
Exchange8 https://www.exchange8.cz/ (Žatecká 8, Praha 1) - Between Metro Staroměstská and Old Town Square
ProvaznickáExchange (Provaznická 386/1, Praha 1) - Near Metro Můstek at Wenceslas Square (yellow B line exit)
ČD (České dráhy) exchange - inside the Main Railway Station (Hlavní nádraží), lower floor
Alfa Exchange www.alfaprague.cz (Náměstí Republiky 1078/1, Praha 1) - Near the Palladium Shopping Mall and Powder Gate