O.k. I thoght, that you already have some hotels on your mind. City centre is so expensive only because tourist are scared to stay little bit outsite the centre. Subway running all day till midnight, trams even longer. Taxi is not expenisive, if you use company as Citytaxi etc., dont use guys on the street. Ask in hotel for taxi-company number and you will be o.k. Choose location near by transport, best subway and you will have no problem to go to city centre quick. Prague city centre is basically for walking, everything is close. You will have some great time, if you go. Try look on Hotel DUO, Prague 9, Prosek. Looks like lots of young people stay there, maybe prices are good. Transport: 3 bus stops to subway, after 6 station to Wenceslaw square. Total travel time cca 20min.