Prague is still relatively inexpensive in comparison to other European destinations. A rough estimate would be to bring along an extra $1600 to $2000 for spending money.
Bring an ATM card and withdraw money as you need it. That is the best way because you get that days exchange rate and you can withdraw a bit of cash every few days so you do not have large amounts of money lying around, that is what we do and it is the safest and most reliable way. You can bring cash USD and exchange it in Prague but your exchange rate will not be as good. Credit cards are widely accepted.
Inform your bank that you are traveling so that they do not freeze your card!